SO THEN (therefore): Ara oun: 

Vine has a nice summary statement on the Spirit of God observing that...

This section is divided in two parts. Romans 8:1-11 have spoken of the indwelling of the Spirit of God; now follows an unfolding of the operation of the Spirit within us in a twofold way, firstly as to His leading and the effects of our response thereto (Romans 8:12-14); secondly, as to the inward witness given by the Holy Spirit, that we are the children of God, and the effects of this, issuing in our being glorified with Christ (Romans 8:15-27). (Vine, W. Collected writings of W. E. Vine. Nashville: Thomas Nelson )

So (686) (ara) here is used as an inferential particle (so, therefore, now, consequently) marking a transition to what naturally follows from the words preceding.

Then (3767) (oun) is a conjunction which is placed after one or more words in a clause, and expressing either the merely external connection of two sentences, that the one follows upon the other, or also the internal relation of cause and effect, that the one follows from the other.

This could be rendered "wherefore therefore" this combination signifying that Paul is drawing a compelling conclusion. In other words he is using 2 "term of conclusion" to emphasize his conclusion. What Paul will teach in these next two verses is that privilege is associated with responsibility. Given that the Spirit Who resurrected Christ will energize our mortal bodies (Romans 8:11), we are no longer debtors to the evil flesh for the flesh did not purchase our new life in Christ. To the contrary we are now debtors to the Spirit Who has given us new life in Christ. Believers are under a "new management" team so to speak,  to Whom they are now obligated.

Paul is saying in essence, "therefore on the basis of your magnificent privilege and provision for victory over sin that you possess by virtue of the indwelling Spirit, you now have a responsibility or obligation to fulfill." Some commentaries feel Paul is drawing together all the truths of a believer he has taught about the believers new position and power since Romans 6, which is possible but is certainly not the immediate context. Others feel that Paul is summing up all the truths of a believer's position and privilege since Romans 8:1 and that is certainly a reasonable thought. Again in the immediate context (Romans 8:10; 8:11) Paul has just explained the great truth that Christ is in our dying bodies and the same Spirit Who God used to raise Christ from the dead indwells us to give life to our dying bodies. On the basis of these great truths he exhorts (and warns) believers about how such privileged people should now live out the remainder of their life.

Hendricksen observes that...

At this point there is a transition from exposition to exhortation; from concentration on blessings bestowed by the Giver, to focusing on the obligation incurred by the recipients, including Paul himself. However, the recipients are by no means represented as being able to act independently. Salvation is not a 50–50 affair. It is God’s gift from start to finish. It is by the Spirit that God’s children must put to death the disgraceful deeds of the body (verse 13), that they are being led (Romans 8:14), and are being moved to cry, “Abba!” (Romans 8:15). It is from the Spirit that they receive the assurance that they are indeed children of God (Romans 8:16). But all this does not take away the fact that the recipients of these favors must go into action. They have an obligation to perform; nevertheless, cannot do this in their own power. How then? As already indicated, “by the Spirit,” and see note Philippians 2:12, Philippians 2:13.(Hendriksen, W., & Kistemaker, S. J. New Testament Commentary Set, 12 Volumes. Grand Rapids: Baker Book House)

Godet writes that that "so then"

refers to the thought of the preceding passage: “Since the Spirit has set you free from the law of sin and death, do not replace yourselves under this curse.”

S Lewis Johnson feels that...

The resurrection is a motive for the life of holiness, and that is the inference the apostle draws from the preceding. (Romans 8:5-17)

In short, Paul is drawing a conclusion and making an application based on the truth about the believer's position before God (In Christ - see notes Romans 8:10) and possessions (Indwelling Holy Spirit - see notes Romans 8:9) which are graciously provided to enable one to live a life of godliness.  "So then" let the power of all these mighty truths govern our thoughts and direct our actions.

Implicit in the teaching in Romans 8:12-13 is the truth that every regenerate individual can live, at times but never as a lifestyle, according to the flesh. Why else would Paul say a believer is no longer obligated to the flesh unless such an obligation were possible? (see also similar reasoning in Paul's command not to let sin reign in our mortal bodies, etc - notes on Romans 6:12;  6:13 - these verses imply that although Sin has been "placed in neutral gear" so to speak, as when a car is idling, this evil disposition can still be engaged into gear and exert it's evil affect over a believer's mortal body.)

Remember that all biblical exhortations and commands to believers are based on the promises they already have from the Lord. Peter makes it clear in his beautiful introduction to his second epistle writing...

that His (God and Jesus our Lord) divine power has granted to us everything pertaining to life and godliness, through the true knowledge of Him who called us by His own glory and excellence. For by these He has granted to us His precious and magnificent promises, in order that by them you might become partakers of the divine nature, having escaped the corruption that is in the world by lust. (see notes 2 Peter 1:3, 2 Peter 1:4)

Without these provisions God graciously provides, believers would be unable to fulfill His commands. The point is that "I by myself can't live this supernatural Christian life". God never said I could. But God can and He said He would live in and through us. Paul put is this way in Galatians in the context of describing his death to the law and all self efforts to attain the righteousness that God requires writing

"I have been crucified with Christ; and it is no longer I who live, but Christ lives in me; and the life which I now live in the flesh I live by faith in the Son of God, who loved me, and delivered Himself up for me." (see detailed note on Galatians 2:20)

We need to acknowledge the sense of our own inadequacy and of His all sufficient adequacy.

This principle flows through both the Old and New Testament. For instance, the children of Israel were not commanded to take possession of the Promised Land until it was promised to them by God and they were prepared by Him to conquer it.

In this letter to Rome, Paul’s primary exhortations begin in Romans 12 (see note Romans 12:1) but only after he expounded on the position and power of each believer now to carry out these exhortations supernaturally, indwelt, controlled and empowered by the Spirit of Christ.

In Ephesians Paul first gives three chapters explaining "every spiritual blessing in the heavenly places in Christ" (see sermon note ) Then just before his beautiful doxology at the end of chapter 3 Paul prays

that He would grant you, according to the riches of His glory, to be strengthened with power through His Spirit in the inner man; so that Christ may dwell in your hearts through faith; and that you, being rooted and grounded in love, may be able to comprehend with all the saints what is the breadth and length and height and depth, and to know the love of Christ which surpasses knowledge, that you may be filled up to all the fulness of God. (see sermon notes on Ephesians 3:16; 3:17; 3:18; 3:19)

Only then does Paul exhort believers to walk a supernatural walk (lifestyle)

“in a manner worthy of the calling with which you have been called, with all humility and gentleness, with patience, showing forbearance to one another in love, being diligent to preserve the unity of the Spirit in the bond of peace.” (see sermon notes Ephesians 4:1; 4:2; 4:3).

Similar patterns of first the believers position and possessions in Christ followed by the call to put this into practice in the power of the Spirit.

F L Godet writes that...

The life of the Spirit is not realized in the believer without his concurrence (agreement or union in action) merely from the fact that the Spirit has once been communicated to him. There is needed on man's part a persevering decision, an active docility (easily taught, led and managed) in giving himself over to the guidance of the Spirit. For the guidance of the Spirit tends constantly to the sacrifice of the flesh and if the believer refuses to follow on this path, he renounces the life of the Spirit and its glorious privileges. (Godet, F L: Commentary on Romans. Kregel. 1998)

BRETHREN, WE ARE (continually) UNDER OBLIGATION: adelphoi, opheiletai esmen (1PPAI): (Ro 6:2-15; Ps 116:16; 1 Corinthians 6:19,20; 1 Peter 4:2,3)

Notice that Paul uses "we" here which links him clearly with his readers, whom he addresses as brethren.

Moule introduces this section observing that...

St. Paul begins with Holiness viewed as Duty, as Debt. He has led us through our vast treasury of privilege and possession. What are we to do with it? Shall we treat it as a museum, in which we may occasionally observe the mysteries of New Nature, and with more or less learning discourse upon them? Shall we treat it as the unwatchful King of old treated his splendid stores, making them his personal boast, and so betraying them to the very power which one day was to make them all its spoil? No, we are to live upon our Lord’s magnificent bounty — to His glory, and in His will. We are rich; but it is for Him. We have His talents; and those talents, in respect of His grace, as distinct from His “gifts,” are not one, nor five, nor ten, but ten thousand — for they are Jesus Christ. But we have them all “for Him.” We are free from the law of sin and of death; but we are in perpetual and delightful debt to Him who has freed us. And our debt is — to walk with Him. (Moule, C. G. The Epistle of St Paul to the Romans. Ages)

Brethren (80) (adelphos from a = denoting unity + delphús = womb) means literally one born from same womb and so describes males having the same father and mother. Here Paul uses adelphos figuratively to describe the close association of persons having a well-defined membership, specifically referring here to fellow believers in Christ, and composing His body, the church.

Godet writes that...

When saying: we are under obligation, literally debtors, Paul meant to continue in the words: to the Spirit, to live according to Him. As soon as the Spirit comes to dwell in our heart, we owe to Him, ourselves, and a life wholly conformed to His wishes. But the apostle breaks off his sentence to set aside the opposite supposition, one unfortunately which cannot be passed over in silence, and he makes haste to add: not to the flesh. “The natural man,” Hofmann observes, “imagines that he owes it to his flesh to satisfy it.” The care of his person, from the most earthly point of view, appears to him the first and most important of his obligations. Now it is this tendency which is combated by the Spirit as soon as He takes possession of us (Gal 5:17). This is the debt which should neither be acknowledged nor paid. The apostle says why in the following verse. (Godet, F L: Commentary on Romans. Kregel. 1998)

Matthew Henry adds that...

We are not debtors to the flesh, neither by relation, gratitude, nor any other bond or obligation. We owe no suit nor service to our carnal desires; we are indeed bound to clothe, and feed, and take care of the body, as a servant to the soul in the service of God, but no further. We are not debtors to it; the flesh never did us so much kindness as to oblige us to serve it. It is implied that we are debtors to Christ and to the Spirit: there we owe our all, all we have and all we can do, by a thousand bonds and obligations. Being delivered from so great a death by so great a ransom, we are deeply indebted to our deliverer.

Paul amplifies why we are obligated to the Spirit writing...

Or do you not know that your body is a temple of the Holy Spirit who is in you, whom you have from God, and that you are not your own? For you have been bought with a price: therefore glorify God in your body. (1Cor 6:19-20)

Under obligation (3781) (opheiletes from opheílo = owe, conveying the basic meaning of owing a debt)  means one who owes another (of one who owes another money) having a strong moral obligation and personal duty) means a debtor, one who is bound by some duty, one who owes anything to another. It can refer to a literal debt  (see Mt 18:24 below) or as used here in Romans 3 times (see below) by Paul figuratively to refer to a personal, moral obligation in contrast to that which is a necessity (which is the Greek verb dei = Click word study of dei) as dictated by the nature of the situation (such as we must eat, we must sleep. We are no longer debtors to the flesh -- what we once were no longer has any claim on us.

Opheiletes can describe one who has committed a misdeed and owes it to the law to make it right - in such case this person is called a guilty person, an  offender or a sinner (see Lu 13:4 below). It is one who has not yet made amends to whom he has injured. For example, it describes one who owes God penalty or whom God can demand punishment as something due (eg, a sinner)

Richards writes that words in the opheilo word group (including opheiletes)...

Words in this group originally expressed the idea of a legal or personal obligation. The Greeks had both financial and, later, moral obligations in mind when they used this term. (Richards, L O: Expository Dictionary of Bible Words: Regency)

NIDNTT adds that...

The word-group formed from the stem opheil- belongs originally to the sphere of law.

Opheilo, attested since Mycenaean Greek., means:
(a) when linked with an object, to owe someone something, e.g. money, a loan (cf. Egyptian sources in BGU III, 846, 16; IV, 1149, 35); and  (b) with an infinitive, to owe in the sense of being indebted (Plato, Leg. 4, 717b).

An opheiletes is (a) a debtor (Plato, Leg. 5, 736d); (b) someone who is under an obligation to achieve something (not found in this sense in the LXX).

Opheile (rare, and not in the LXX) and the more common opheilema (in the LXX only in Deut. 24:10; 1 Macc. 15:8) denote a debt, particularly of a financial nature,

Ophelon, originally an aorist participle of opheilo with the addition of estin (is), became the set expression for the optative “O, that”, “would that”, “if only” (cf. Epict., Dissertationes 2, 22, 12).

2. Alongside financial there are also moral obligations in respect of people or of state laws. Thus a culprit is often punished by being required to pay compensation to the injured party (Plato, Cra. 400c: until he has made the necessary payments). Infringement of divine regulations and thanks which must be rendered in return for benefactions of the gods also make men debtors, in requiring from them some cultic penance or act. Thus in Plato, Phaedo 118, the dying Socrates says: “We owe Asclepius [the god of healing] a cock.” Correlates and formations from opheilo thus contain both the negative component of debt and the positive one of obligation. (Brown, Colin, Editor. New International Dictionary of NT Theology. 1986. Zondervan)

The TDNT has a nice summary of the root word opheilo noting that it is...

Etymologically obscure, this word means “to owe someone something,” e.g. loans, debts, sums, or rents. The things owed may be spiritual, and the word is also used with the infinitive for “to be under obligation to,” “to have to.” The word is common in respect of revenge or law. Transgressors are in debt to injured parties. Secular and sacral penalties are owed. God’s goodness also makes people debtors. This gives rise to the idea of moral obligation. (Kittel, G., Friedrich, G., & Bromiley, G. W.  Theological Dictionary of the New Testament. Eerdmans)

Here are the 7 NT uses of opheiletes (not in the Septuagint - LXX) translated culprits, 1; debtors, 1; indebted, 1; owed, 1; under obligation, 3...

'And forgive us our debts, as we also have forgiven our debtors (opheiletes). (see notes Matthew 6:12) (Comment: Those who sin against us are viewed as our debtors whom we are to forgive just as God Himself forgave us a sin debt we could never repay).

"And when he had begun to settle them, there was brought to him one who owed (opheiletes) him ten thousand talents (such a large amount in essence it cannot be paid = Jesus is teaching about man's dependence on and responsibility to God who will settle accounts with His servants - ultimately only through the compassion of the creditor could such a high debt be remitted - and so men should forgive one another debts which by comparison are miniscule instead of insisting on their "legal" rights!). (Mt 18:24)

"Or do you suppose that those eighteen on whom the tower in Siloam fell and killed them, were worse culprits (opheiletes; KJV renders it "sinners") than all the men who live in Jerusalem? (Luke 13:4)

"I am under obligation both to Greeks and to barbarians, both to the wise and to the foolish." (see notes Romans 1:14)

Yes, they were pleased to do so, and they are indebted to them. For if the Gentiles have shared in their spiritual things, they are indebted to minister to them also in material things. (see notes Romans 15:27)

And I testify again to every man who receives circumcision, that he is under obligation to keep the whole Law (the law is viewed as a unit, thus obedience to it cannot be selective). (Galatians 5:3)

Now we are indebted to the Holy Spirit for all He has done and will do in and through us. But don't be confused...we cannot "earn" God's favor. God gives us the power (His indwelling Spirit - see notes Romans 8:9, 8:10, 8:11) to "pay Him back" with works of righteousness (not works done in the flesh...these are dead works) done out of heart of obedience (see note Romans 6:17)...now we offer back to Him our obedience (1Sa 15:22) realizing that even that would not be possible unless He had changed our hearts from their godless, hostile, helpless, sinful (Romans 5:6-10-notes) condition to a new heart with new motivation that He provides that now we might be pleasing to Him.

NOT TO THE FLESH TO LIVE (habitually, continually) ACCORDING TO THE FLESH: ou te sarki tou kata sarka zon (PAN):

Not (ou) is the Greek word indicating absolute negation. The point is that believers have been set free from this harsh master personified as the Flesh. Prior to our new birth we were in fact "obligated" to the flesh, and had to obey it's desires and wishes! (eg, cp notes on "enslaved to various lusts and pleasures" Titus 3:3) There is now no obligation to the flesh. In fact now Paul commands believers...

Therefore (because believers now are "dead to sin, but alive to God in Christ Jesus" see note Ro 6:11) do not let sin reign in your mortal body (present imperative is a command preceded by a negative = "Stop letting the Sin continue to reign in your physical body!") that you should obey its lusts 13 and do not go on presenting (also present imperative with a negative) the members of your body to (the) Sin (which continues to want to "usurp" the throne which now belongs to Christ Jesus alone) as instruments of unrighteousness; but present (aorist imperative = Command to do this now and do it effectively!) yourselves to God as those alive from the dead, and your members as instruments of righteousness to God.  (see notes Romans 6:12; Romans 6:13)

Believers now are debtors to and in that sense obligated to righteousness (not a legalistic obligation but a love obligation that comes from our new heart and the new covenant). It is not enough that we have received the Holy Spirit (which we have). It is now our moral/ethical obligation (motivated by our remembrance of the love of God for us as evidenced by the Cross and the love and anticipation we have of our Lord's imminent Second Coming) to walk not according to the flesh but according to the Spirit. (see notes Galatians 5:16; Galatians 5:17; Galatians 5:18)

Pritchard writes that...

Why don’t we owe anything to the flesh? One, because we’ve been set free from the power of the flesh. We are no longer "in the flesh" but "in the Spirit." The flesh once controlled us, but now we are free. Two, because the flesh does us no good. Consider the "ministry" of the flesh: 1. It tempts us to do evil.  2. It pulls us away from God. 3. It wars continually against the Holy Spirit. You don’t have to live in the flesh any more because you don’t owe your flesh anything. (Romans 8:5-17: Life in the Spirit)

Warren Wiersbe explains the debt this way...

Our obligation is to the Holy Spirit. It was the Spirit who convicted us and showed us our need of the Savior. It was the Spirit who imparted saving faith, who implanted the new nature within us, and who daily witnesses within that we are God’s children. What a great debt we owe to the Spirit! Christ loved us so much, He died for us; the Spirit loves us so much, He lives in us. Daily He endures our carnality and selfishness; daily He is grieved by our sin; yet He loves us and remains in us as the seal of God and the “down payment” (“earnest,” 2 Cor. 1:22) of the blessings waiting for us in eternity. If a person does not have the Spirit dwelling within, that person is not a child of God. (Wiersbe, W. W. Wiersbe's Expository Outlines on the New Testament. Wheaton, Ill.: Victor Books)

Morris explains that...

Paul is laying it down firmly that for Christians the flesh has no rights; as Earle puts it, “we owe the flesh nothing”. The way Paul puts it, “we are debtors, not to the flesh—”, leads us to look for “but to the Spirit” or for some other expression to indicate where our debt lies, “but this is elegantly left to be understood” (Bengel). The characteristic life of the Christian owes nothing to the flesh, though we should not ignore the force of this warning about “the flesh” in a letter to Christians. “The flesh” is not eradicated but is an ever-present reality. Paul goes on to explain that to be indebted to the flesh means “to live according to the flesh”. This is not an option for the believer. (Morris, L. The Epistle to the Romans. W. B. Eerdmans; Inter-Varsity Press)

As Moule says...

For a moment he turns to say what we owe “no” debt to even “the flesh,” the self-life. But it is plain that his main purpose is positive, not negative. He implies in the whole rich context that we are debtors to the Spirit, to the Lord, “to walk Spirit-wise.” (Moule, C. G. The Epistle of St Paul to the Romans. Ages)

Not (3756) (ou) defines absolute negation. Paul's point is strong - the believer is no longer under moral obligation or debt to the flesh, that evil disposition inherited from Adam which is opposed to God and can do absolutely nothing to please God. The old, sinful nature may present its desires, even its demands, based upon our past, but we are now under no obligation to cooperate!

Live (2198) (zao) in the present context refers to living one's life. The important truth Paul brings out with the use of the present tense is that believers are no longer those who live continually under the dominion of the evil flesh nature. Sure, none of us have arrived so to speak (that is called "glorified"!) and we are "prone to wander" as the hymn writer laments. However our wandering is for a moment or even a season but not for a lifetime and not as our continual habitual practice.

According to (2596) (kata) can mean "down" and in this context can picture a person living "down under" the dominion of the flesh.

Flesh (4561) (sarx) as used in the present context is that evil part of man inherited from Adam and unfortunately still having residence in a believer's mortal body. The flesh is at war with God and is not able to be controlled or changed by the Law. In fact when a believer now attempts to keep the law (or any form legalism might take in one's life), he or she is only giving opportunity to the old flesh nature to work! When a believer begins to live by Law they begin to depend on their own strength (the evil flesh nature) and they are left to get by without God’s supply or the Spirit's enablement. And the efforts of the flesh can never accomplish what faith can accomplish through the Spirit. In short, the old nature cannot be controlled by Laws or rules and eventually breaks out thus explaining why legalistic religious groups are so prone to fights and divisions. There are many popular false religious teachers who promulgate a religion that pampers the old flesh nature and feeds the ego (which accounts to a large extent for their popularity).

As J Vernon McGee reminds us...

My friend, the flesh—and we all have it—is a low-down, dirty rascal. And we don’t owe it anything. (McGee, J V: Thru the Bible Commentary:  Nashville: Thomas Nelson)

MacDonald echoes these thoughts writing that the flesh is nothing but...

The old, evil, corrupt nature has been nothing but a drag. It has never done us a bit of good. If Christ had not saved us, the flesh would have dragged us down to the deepest, darkest, hottest places in hell. Why should we feel obligated to such an enemy? (MacDonald, W., & Farstad, A. Believer's Bible Commentary: Old and New Testaments. Nashville: Thomas Nelson)

The flesh is recognized in a believer when he or she is living for self and seeking