Wherefore (term of conclusion) - This term includes the entire section from (Ro 14:1-15:6), and here sums up the major emphasis of this section. In immediate context because the purpose of being of one accord is to bring glory to God the Father, we are therefore to accept one another that this glorious effect might be realized.

Stedman asks the question...

"Do you remember the prayer of the little girl who once prayed, "Lord, make the bad people good and the good people nice"? This is really the subject of Ro 14-15 -- making good people nice! One of the problems of the Christian church is that we may be quite correct in our doctrine and practice, but very irritating about it. How do you live with people like that? That is the problem of these chapters, and it is a problem that abounds everywhere -- how to live with other Christians who persist in looking at things differently than you do."

ACCEPT ONE ANOTHER: Dio proslambanesthe (2PPMM) allelous:  (Mt 10:40 Mk9:37 Lk 9:48)

Spurgeon...
 

If the Lord Jesus has indeed received us, and bears with our weaknesses and follies, well may we have patience with one another, and show pity to each other's infirmities.

Christ did not receive us because we were perfect, because he could see no fault in us, or because he hoped to gain somewhat at our hands. Ah, no! but, in loving condescension covering our faults, and seeking our good, he welcomed us to his heart; so, in the same way, and with the same purpose, let us receive one another.

Middletown Bible - The Law of Love (Romans 14:1-15:3) - For further help in understanding how to live so as to not cause a brother to stumble, see paper entitled, "Guidance: 67 Biblical Tests to Use in Deciding Upon a Course of Action."

Accept (proslambano) (4355) Paul commands us to make this our habit, our lifestyle. (Study the "one anothers" - most positive, some negative)

Proslambano is always in the middle voice in the NT which conveys the idea that the subject not only initiates the action but also participates in the results of that action. Paul is calling for a "reflexive" (you yourselves demonstrate) a whole hearted attitude and action.

So Paul says continually draw one another to yourself (that's the idea of the middle voice).

Give them a warm welcome.

Grant them access to your heart.

Take them to yourself.

Treat them as the closest of friends with the most caring kindness.

Believers are to receive one another in the closest of bonds. Let your love be without hypocrisy (Ro 12:9-note) and demonstrate a genuine heartfelt acceptance. Take them into friendship. And remember that in the final analysis to accept one another is actually to accept Christ Himself (Mt 10:40)

JUST AS CHRIST ALSO ACCEPTED US TO THE GLORY OF GOD: kathos kai o Christos proselabeto (3SAMI) humas eis doxan tou theou:

It should not be too difficult to extend the hand of friendship to one who is loved by the One we honor and worship and Who first loved us.

The idea should be "Any friend of His is a friend of mine."

Accepted - The same verb proslambano in (Ro14:3-note) describes God accepting the strong brother.

What were we like before Christ accepted us? Romans 5 gives us a "thumbnail sketch" where Paul describes unbelievers as "helpless... ungodly... sinners... enemies". (Ro 5:6, 7-note, Ro 5:8, 5:9-note, Ro 5:10-note)

So what excuse do we have not to accept another, no matter what the differences we might have with them in the arena of non-essentials. Jesus accepts sinners impartially (Ro 2:11-note).

Jesus accepts sinners to the glory of God. God established His eternal plan of redemption to glorify Himself. Everything He does is to His glory, and everything His children do should be to His glory.

Leenhardt writes

Nothing glorifies God as much as the unity of His children, which alone is in harmony with His essential will of love.

Believers are to receive one another even as Christ has received us. No person is too prejudicial or critical, too weak or base for us to receive. The point is that Christ received and ministered to the most prejudicial and judgmental (the Jews), sinful and base (the Gentiles) people in the world; therefore, believers can receive one another (Gal 3:28,  Ep 2:11-22-note). No one is too far gone to be rejected and cast away.

Ray Stedman has some practical insights writing:

"Now, that is the end and aim of human life, to "glorify the God and Father of our Lord Jesus Christ." Every Christian heart wants that. Is there any Christian who, deep down in his heart, does not want the glory of God in his life? Well, then, the way to produce it is to live in harmony with one another. That is the procedure. It says that even though Christians: Hold different points of view (they do not come to some universal agreement on doctrine -- that isn't what is necessary), despite this, they can be so interested in one another and so concerned about one another that they can live in harmony, and the result is that they glorify God. What is the key to it? It is tucked away in one little phrase which, if you miss it, you will want to live in harmony with others but you will find yourself quite incapable of doing it. The key is this little phrase, "in accord with Christ Jesus." (Ro 15:5b) You see, when Paul mentions that Christ did not please Himself, he is not holding Him up as an example to follow, he is lifting Him up as a life to appropriate! That is the big difference! We are not just to try our best to live the same way, because we CAN'T. Our own natural inclinations of self-pleasing rise up and refuse to permit us to do that. But we must never forget that these exhortations that we read in these last five chapters of Romans rest solidly and squarely on the teachings of Ro 5, 6, 7, 8. It is back there that we learn that God has done something about this old Adamic life which naturally moves to evil -- has cut it off, and has given us, in place of it, the indwelling life of the Son of God Himself, and it is His purpose and desire to live that life again through us! In the doing of it, we find it quite possible to do all that He does because He does it through us. This is what it is speaking of here when it says, "in accord with Christ Jesus," that is, in fellowship with him. Biting your lip, and trying to keep your temper is not the SECRET of living with difficult people; that is never it. The SECRET is a thankful heart which continually looks up to the Lord Jesus. It says, "Thank You, Lord, for the quietness and the calmness, the purity and the love which is available to me through You continually. Thank You." A thankful heart and an obedient will that seeks to please another for his own good is the SECRET of living with difficult people. You try that and see if it doesn't work." (Power to Please)

John MacArthur adds that

Paul does not mention specific types of believers in this verse. He is speaking to the strong and to the weak, to Gentiles and to Jews. All believers are called to accept one another. He is not simply speaking of accepting new believers into our church fellowship, although that would certainly be included in this admonition. He is calling on all Christians to accept one another in the fullest and deepest sense, to treat each other with love and understanding, just as Christ also accepted us. If the perfect, sinless Son of God has accepted us into God’s divine family, how much more should we be willing to accept each other, despite the fact that we all still carry sinful trappings from our old, unredeemed flesh. (MacArthur, J: Romans 9-16. Chicago: Moody Press or Logos)

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Include The Unaccepted (Romans 15:4-13) - Christ's followers are called to live an "inclusive" lifestyle. By this I mean that we are instructed in God's Word to reach out to those who might not be accepted by others. But we get so comfortable with our church friends that we may feel it's too much to ask.

In The Covenant Home Altar, Erika Carney wrote, "When I was growing up, my mom was deeply involved with the various activities I participated in. She would often tell me, 'Make sure Heather feels included,' or 'Ask Julie to be your partner,' or 'Invite Kristen to sit with you and your friends.' I sometimes dreaded hearing those words. What if I just wanted to be with my friends? Why should I have to make sure that other people were having fun?"

Erika's mother was teaching her daughter to obey the principle taught in Romans 15:7. We are to welcome and accept Christians who may not feel comfortable in our particular group.

Some people are excluded because they are withdrawn, angry, or defensive, or because they seem strange or demanding. But these things shouldn't matter. We are to receive fellow believers "just as Christ also received us, to the glory of God" (Ro 15:7). — David C. Egner

Some people can be difficult to love
And so we do not even try to care,
But God says, "Love them just as I've loved you--
You'll bring Me glory as My love you share." --Cetas

We care for others because God cares for us.

FOR I SAY THAT CHRIST HAS BECOME A SERVANT TO THE CIRCUMCISION ON BEHALF OF THE TRUTH OF GOD: lego (1SPAI) gar Christon diakonon gegenesthai (RPN) peritomes huper aletheias theou: (Mk 10:43, Mt 20:26, Mt 23:11)

Paul has explained that believers are to accept one another just as Christ accepted each one of us and now he elaborates on what Christ's acceptance of us entails -- in a nutshell, as menial (diakonos) (see in depth discussion of related word diakonia) servant, of both Jews and Gentiles.

Spurgeon...
 

It was to Abraham and his descendants that the promise was made that, in him, and in his seed, all the nations of the earth should be blessed. So our Lord came, as a Jew, to be "a minister of the circumcision." Let us never forget that he came to those whom we are apt to forget; and, peradventure, even to despise, "to confirm the promises made unto the fathers.'"

Become (ginomai) - Paul used perfect tense for become indicating a permanent state -- Christ continues in his capacity as "a servant of circumcision". His function as Jewish Messiah is permanent.

Paul is explaining the breadth of Christ's acceptance of us. Christ received the Jews and ministered to them and He did so for the truth of God -- to confirm and fulfill the promises God made to the patriarchs. Did He accept them because they accepted Him? In fact they did not receive Him! (Jn 1:11) and yet Christ came to them and received and ministered to them. Why? For the sake of the truth of God—to fulfill the OT promises made in the Word of God for "not one word has failed of all His good promise" (1Ki 8:56 cp Josh 23:14 Nu 23:19)

Circumcision (4061) (peritome [word study] from perí = around + témno = cut off) refers literally to cutting and removal of the foreskin. As discussed below both the Old and New Testament also use the concept of circumcision in a figurative or metaphorical sense. (See also exposition of Scriptures on Circumcision)

The Jews took great pride in circumcision which became a badge of their spiritual and national superiority thus fostering a spirit of exclusivism instead of a missionary zeal to reach out to the Gentile nations as God had intended. A daily prayer of strict Jewish males was to thank God that he was neither a woman, a Samaritan, nor a Gentile. Gentiles came to be regarded by the Jews as the “uncircumcision,” a term of disrespect implying that non-Jewish peoples were outside the circle of God’s love. Paul had already dealt with genuine (internal, heart) circumcision and contrasted this with the external ritual (Ro 2:28, 29-note).

TO CONFIRM (make firm, strengthen, guarantee) THE PROMISES GIVEN TO THE FATHERS: eis to bebaiosai (AAN) tas epaggelias ton pateron:  (Ro9:4,5)

How did Messiah confirm the promises to the patriarchs? Clearly by being the ultimate fulfillment of those OT promises (To Abraham Ge 12:3, 17:7, 18:19, 22:18, Acts 3:25) (To Isaac Gen 26:3, 4) (To Jacob Ge 28:13, 14, 15, 46:2, 3, 4).

Paul made it clear both that the covenant promises of God are permanent and that they find their fulfillment in Christ.

Paul's point is that herein lies our motivation to accept one another, to bear one another's weaknesses and not to please our self but build up one another (Ro 15:1, 2-note).

The believer has the greatest pattern imaginable: Christ Himself.

AND FOR THE GENTILES TO GLORIFY GOD FOR HIS MERCY: ta de ethne huper eleous doxasai (AAN) ton theon:  (John 10:16)

Spurgeon...
 

And that the Gentiles might glorify God .for his mercy; as it is written: For this cause I will confess to thee among the Gentiles, and sing unto thy name. And again he saith, Rejoice, ye Gentiles, with his people. And again, Praise the Lord, all ye Gentiles; and laud him, all ye people. And again, Esaias saith, There shall be a root of Jesse, and he that shall rise to reign over the Gentiles; in him shall the Gentiles trust.

There were plain indications, in the Old Testament, that the blessing was meant for the Gentiles as well as for the Jews; but, still, it was made known to the Jews first, and we must never forget that.

The saved Jew primarily praises God for His truth and the saved Gentile primarily praises God for His mercy. Christ received the Gentiles and ministered to them. He came to bring them the mercy of God, that they might praise God for His mercy. And don't forget the context -- Jesus received and ministered to the Gentiles when they were living the most licentious and indulgent life-style imaginable (Ro 1:18-32-note).

God had made covenants only with Israel (Ro 9:4-note), not with the Gentiles (Ep 2:12-note), so God had no covenantal promises to confirm with the Gentiles. Any spiritual blessings that come to the Gentiles spring solely from the mercy of God. Nevertheless God eternally purposed to bless the Gentiles spiritually through the Lord Jesus as their Messiah and through His covenants with Israel (Ge 12:3 cf. Jn 4:22).

It is no accident that Paul's quotations in (Ro 15:8, 9, 10, 11, 12) are taken from the three divisions of the OT, the Law, the Prophets, and the Psalms (Lk 24:44) and from three great Jewish heroes: Moses, David, and Isaiah.

A progression of thought can be traced through the four quotations. In the first, David praised God among the Gentiles (Ro 15:9); in the second, Moses exhorted the Gentiles, Rejoice . . . with His people (Ro15:10); in the third the psalmist commanded the Gentiles to praise the Lord (Ro 15:11-note; cf. Ro 15:7) and in the fourth, Isaiah predicted that the Gentiles will live under the rule of the Root of Jesse (the Messiah) and they will hope in Him (Ro 15:12-note).

AS IT IS WRITTEN THEREFORE I WILL GIVE PRAISE TO THEE AMONG THE GENTILES: kathos gegraptai (3SRPI) dia touto exomologesomai (1SFMI) soi en ethnesin:

Written (1125) (grapho [word study]) from root graph- = primarily means to scratch on or engrave as on an ornament, reports, letters, etc; English = graph, graphic, etc) means to engrave or inscribe with a pen or stylus characters or letters on a surface which can be wood, wax, metal, leather, stone, parchment, dirt (John ), paper, etc. (Click to review all 191 uses of grapho in the NAS)

Grapho is in the perfect tense meaning that this "has been written and stands written".

To illustrate that it has always been God’s plan to bring Gentile into His kingdom, Paul appeals not to his great learning but to the unchanging Word of God in the Old Testament, possibly to soften the prejudice of Christian Jews against Christian Gentiles as their own Scriptures demonstrate that the inclusion of Gentiles was neither a divine nor a human afterthought.

This first OT quotation is from the Greek Septuagint of (2Sa 22:50, Ps 18:49-note) where David is praising God for giving him victory over his enemies and for making the nations subject to him. David goes on to say will sing hymns of praise to the Lord among the Gentile nations, so that the Gentiles may know the true God and join in the praise.

Paul sees in this Psalm as a clear indication of God’s plan to include the Gentiles in the Messiah’s people. Israel was to have been the instrument through which God's redemptive work would extend to the Gentiles.

AND I WILL SING TO THY NAME: kai to onomati sou psalo (1SFAI):

Sing (5567) (psallo) is derived from psáo which means to touch lightly, twang or snap.

AND AGAIN HE SAYS REJOICE O GENTILES WITH HIS PEOPLE: kai palin legei (3SPAI) euphranthete (2PAPM) ethne, meta tou laou autou: