NOW TO HIM WHO IS ABLE : To de dunameno (PPPMSD):  (Ro 14:4; Acts 20:32; Ephesians 3:20,21; 1 Thessalonians 3:13; 2 Thessalonians 2:16,17; 3:3; Hebrews 7:25; 1 Peter 5:10; Jude 1:24,25)

Paul praises God for the gospel that establishes men.

Him Who is able - God is continually (present tense) able. How able is He? Paul elaborates in Ephesians 3 exalting...

Now to Him Who is able to do exceeding abundantly beyond all that we ask or think, according (proportionate to His power not a portion of it!) to the power that works within us, to Him be the glory in the church and in Christ Jesus to all generations forever and ever. Amen. (See notes Ephesians 3:20; 3:21)

Able (1410) (dúnamai) (See study of related word dunamis) has sufficient power by virtue of His own inherent ability to establish those who trust in Him according to the true gospel that Paul, and every true preacher and teacher, have clearly set forth.

MacArthur notes:

There is a remarkable relationship between the last three verses of Romans and the first eleven. In (see note Romans 16:25)] Paul speaks of Him who is able to establish you, and in (see note Romans 1:11) he talks of his readers being established. He speaks of my gospel, and in (see note Romans 1:1) of “the gospel of God.” He speaks of the mystery of God which has been kept secret for long ages past, and in (see note Romans 1:2) of the gospel “which He promised beforehand.” He mentions the preaching of Jesus Christ, and in Romans 1:3 the gospel concerning God’s Son. He speaks of the Scriptures of the prophets, just as he does in (Romans 1:2). He speaks of the gospel being made known to all the nations, leading to obedience of faith, and in (see note Romans 1:5) of bringing “about the obedience of faith among all the Gentiles.” (MacArthur, J: Romans 9-16. Chicago: Moody Press)

TO ESTABLISH YOU: humas sterixai (AAN): (2Th 2:17, 3:3, Ro 1:11, 1Th 3:2 3:13, Ja 5:8 1Pe 5:10 2Pe 1:12 Rev 3:2, Lu 22:32 cp Ro 5:2)

"Now to him who is able to set you on your feet" (Phillips)

Establish (4741) (sterizo from histemi = to stand) means to put or place something firmly in a location, cause it to be fixed or establish it in a place. (Click word study of sterizo)

For example in Luke Jesus explains the fixed state of the chasm that separated the two sides of Sheol declaring...

'And besides all this, between us and you there is a great chasm fixed (sterizo) in order that those who wish to come over from here to you may not be able, and that none may cross over from there to us.' (Luke 16:26)

Sterizo then means to place something firmly in a location, so as to make it firm and stable. In the present context sterizo refers to steadfastness in mind, reflecting a mental state that is settled and firmly rooted in the truth of the "gospel and the preaching of Jesus Christ".

Denney remarks that sterizo...

takes us back to the beginning of the epistle (see note Romans 1:11). Paul wished to impart to them some spiritual gift, to the end that they might be established; but only God is able (cf note Romans 14:4) to effect this result. The establishing is to take place ‘according to my gospel:’ in agreement with the gospel Paul preached. When it was achieved, the Romans will be settled and confirmed in Christianity as it was understood by the apostle.

Ray Stedman paints the following picture of "establish":

"Have you ever had the desire to be established? Many people think they are established when actually they are simply stuck in the mud. Most of us think that being established means that all progress ceases. We sit down, camp there, and that is it. In that sense, there are a lot of Christians who are established. But when Paul speaks of our being established, he means putting us on solid, stable ground. Have you ever erected a picnic table and tried to find a place where all four legs touched the ground at the same time? You tried to establish it so that it would not rock, or become shaky, or uncertain. That is the idea that Paul has in mind in this word establish. God wants to bring you and me to a place where we are no longer rocking or shaky or unstable, but solid and secure. The idea is basically what all human beings look for -- an inner security from which you can handle all the problems of life. You become dependable, and have a true sense of worth, so that nothing gets to you, or shakes you up, or throws you off balance. This is the goal of all Christian teaching in the New Testament (and especially the goal of the letter to the Romans) that we believers might be brought to that place of security where we are not shaken by things, so that we do not lose our tempers easily, or get frustrated, angry, resentful or hostile; where we do not scream at our children, or yell at our mates, or get upset at the neighbors. Notice the resource that the apostle counts on to make that happen: "Now to him who is able to establish you..." It is God himself who is responsible for this. You and I are not given the final responsibility to bring this about. Isn't that encouraging? " (Read his full sermon The Great Mystery)

William Newell adds

The Greek word [sterizo] translated establish is used about 10 times in the NT concerning a settled, stable spiritual condition. We find this first in our Lord’s words to Peter: "When once thou hast turned again, establish thy brethren" (Lu 22:32). It includes not only a knowledge of the truth, and a settled persuasion in Christ of that truth; but also obedience in the power of the Spirit, to the truth: "to the end He may establish your hearts unblamable in holiness before our God and Father, at the coming of our Lord Jesus with all His saints" (see note 1Thessalonians 3:13) ; and it also involves our testimony: "establish your hearts in every good word and work" (2 Thes 2:17).  (Romans 16)

Who establishes? God. How? He uses men who preach the Way, the Truth, the Life in clear exposition of the gospel & of Jesus Christ. (cp same thought in Romans 1:11 [see note] where Paul wanted to impart a spiritual gift to them that they might be established, firm in their faith). Preachers are to feed the sheep pure food, which alone can equip & firmly ground them in the truth that alone sets men free & keeps them from being blown about by every wind of doctrine (see note Ephesians 4:14). Saints who are not in the Word of God ("ain'ts') are open to deception. All saints stand in continual need of "every word that proceeds from the mouth of God" (Matt 4:4) to maintain spiritual stability. Compare Paul's prayer for the Ephesian saints (see notes Ephesians 3:16; 3:17; 3:18; 3:19).

ACCORDING TO MY GOSPEL  AND THE PREACHING OF JESUS CHRIST: kata to euaggelion mou kai to kerugma Iesou Christou:  (Ro 2:16; 2 Corinthians 4:3; Galatians 2:2; 2 Thessalonians 2:14; 2 Timothy 2:8) (Acts 9:20; 1 Corinthians 1:23; 2:2; 2 Corinthians 4:5)

My gospel -  Paul says that is what God will use to establish you: "My gospel -- that which was given to me."

Some teachers have mistakenly interpreted these words to suggest that Paul had a unique gospel, and thus they have concluded that the Apostle Paul was giving a special revelation that no one else possessed -- not even Peter, James and John! This is absurd, but sadly has been taught, and people have followed bought into that lie. Paul spoke to such aberrant teaching in First Corinthians writing

"Some of you are following me; some are following Apollos; some are following Cephas, and this is wrong. We are not different; we all have the same gospel. You are making too much of men. The message is always the same," {1 Cor 1:11 ff}.

He rebuked them for tending to divide and to follow certain leaders and teachers. The practical impact is that the test of all true Christian messages is that they be in line with the apostolic writings. The apostles are the ones who tell us the truth about the gospel. That is why we must always check what we hear today that claims to be Christian and see if it fits with what the apostles gave us.

Gospel (2098) (euaggelion) ( Click word study of gospel (euaggelion)

So through the gospel, God is able to establish the minds and hearts of believers in the truth, to settle us, ground us, and make us firm in Him. The only gospel of God that establishes men is the gospel that proclaims Jesus Christ, which is how Paul began this letter

gospel of God which He promised beforehand through His prophets in the holy Scriptures concerning His Son. (see notes Romans 1:1; 1:2)

Theology was not the heart of Paul's gospel. The heart of his gospel was the revelation of a Person, Jesus Himself. All through this letter Paul has emphasized that fact again and again -- everything centers in & on Christ. He is the heart of it all. Therefore a gospel that leaves out Christ is a phony gospel.

ACCORDING TO THE REVELATION OF THE MYSTERY WHICH HAS BEEN KEPT SECRET FOR LONG AGES PAST: kata to euaggelion mou kai to kerugma Iesou Christou, kata apokalupsin musteriou chronois aioniois sesigemenou (RPPNSG): (1 Corinthians 2:7; Ephesians 1:9; Colossians 1:26,27) (Psalms 78:2; Daniel 2:22; Amos 3:7; Matthew 13:17,35; Luke 10:23,24; Ephesians 3:3-5,9,11; 1 Peter 1:10-12,20)

Revelation (6028) (apokalupsis from apó = from + kalúpto = cover, conceal) (Click word study of apokalupsis)  literally pictures an unveiling or uncovering of something hidden, this uncovering providing "light and knowledge" to those who behold it. The idea is exposing to full view what heretofore was hidden, veiled or secret. Revelation is a disclosure. In short what was once unknown, is now revealed.

Mystery(3466) (musterion from mustes = one initiated [as into the Greco-Roman religious "mystery" cults] from mueo = to close or shut) (Click word study of musterion) was a common Greek word which in its classic use referred to the secrets imparted only to initiates in the famous "mystery cults" of that day. In modern vernacular "mystery" continues to imply something hidden & unknown & even unknowable! In contrast, the NT uses musterion to describe truth previously hidden but now divinely revealed.

When Paul uses musterion (and his is the major use 20 out of 27x in NT) he does so to describe a spiritual truth which has been long hidden but has been revealed to man by God in His time.

The "mystery" Paul is referring to is the fact that God intends to unite both Jews and Gentiles into one body (cf "mystery" in [see notes Romans 11:25; 11:26] "...mystery...that a partial hardening has happened to Israel until the fulness of the Gentiles has come in & thus all Israel will be saved...").

Thus as Paul has already taught for the two to become one body in Christ (the "church"), the Jews are now partially blinded for a while, in order to allow the Gentiles to see. This partial blindness of Israel has been going on for about 2,000 years. This mystery of the Gentiles joining the Jews in salvation is also discussed in [see notes Ephesians 3:1; 3:2; 3:3; 3:4; 3:5; 3:6] where Paul explained

that the Gentiles are fellow heirs and fellow members of the body & fellow partakers of the promise in Christ Jesus through the gospel". (see notes Ephesians 3:6)

Ray Stedman adds the following insight to help understand the profound truths of the "mystery":

"The heart of the mystery is given to us in the opening chapter of Colossians. Here is one of the clearest statements on it (See notes Colossians 1:24; 1:25; 1:26; 1:27) "...this mystery among the Gentiles which is Christ in you the hope of glory"). There is the mystery. All that God is, wrapped up in a Person and given to you and to me -- the only hope we have of ever discovering the glory that God intended for us as human beings: Christ in you, the hope of glory....Jesus himself is the mystery (see 1 Tim 3:16). By means of the virgin birth of Jesus, by means of his holy, sinless life, by means of his substitutionary death upon a violent and cruel cross, by means of his startling break-out from the prison of death, and by means of the gift of the Holy Spirit on the day of Pentecost, God has given Jesus -- all that he is and all that he has -- to you and to me. This enables us to do two things: To deny our natural abilities and strengths, and to rely wholly on Jesus' ability & strength -- & thus to live our lives today as though Jesus Himself was living them. That is the mystery. That is the radical, powerful secret of authentic Christianity: Christ in you, the hope of glory. Do you know that mystery? Do you know it, not only in your mind, but do you live it? It is the knowledge of it and the living of it that turns Christianity into an exciting adventure. It may be demanding, it may even be scary, but I can guarantee you one thing: It will never be boring, because the mystery is at work. If you are filled with the secret, the indwelling Christ, it does not make any difference if you are a Jew or a Gentile. All the divisions of class and sex and national origin are eliminated by that secret. It does not make any difference whether you are rich or poor, slave or free, all are one in Christ Jesus by that mystery. And whenever a Christian lives on that basis, really trusting the fact that God is in him through Jesus Christ to be his wisdom, his power, his strength; when he attempts things only on the basis of expecting God to fulfill that promise, and moves out to do things by His grace, he finds himself established. If you want a place of security, it is not going to come by your reckoning on what you can do for God. That will never work. It is going to depend on how much you believe God is ready to do something through you. That is the radical promise....What a change happens when people really capture this and begin to operate on it....by the death of Jesus, God cut off all the natural abilities and strengths that we have, and rendered them worthless. The New Testament teaches us that the flesh cannot please God....Jesus himself, the bread that came down from heaven, is available to us. His strength, His power, operating through the channel of our gifts, can accomplish what we could never do by ourselves. God at work within us; that is the mystery....in every situation throughout the week, every moment of pressure and every demand upon us are simply opportunities to respond by realizing again the validity of the mystery. " Jew & Gentile one in Christ & Christ in us the hope of glory. What a note to end the book of Romans on! (Read his full sermon The Great Mystery) (Bolding added)
 

Ray Stedman makes a piercing comment regarding the revelation of the mystery

Here is the ultimate test of any Christian message: Does it proclaim the mystery? There are thousands of places in this land today where people are meeting, as we are, in Christian churches. They are singing the same hymns we sing, and reading the same Bible, and praising God in the same way. And yet, in thousands and thousands of those churches, there is nothing exciting happening, nothing that reaches out and touches the community. Do you know why? Because the mystery is not being proclaimed. After the early service this morning someone told me of a town in California of about 8,000 people, where there are 22 churches. And, according to this individual, almost all of these churches are lifeless. Nothing is happening because they do not understand the mystery. Here is the heart of the gospel, this amazing mystery. The question we need to ask about any church is,

"Does it ask men and women to live on the basis of that fantastic secret, which was once hidden but is now fully revealed?" (Ibid)  (Bolding added)

Stedman adds that

"It is God who does the strengthening in our lives, and, Paul says, it will always be...by three particular means:

(1) "according to my gospel, and the preaching of Jesus Christ." That is, it will be right along the lines that the Holy Spirit has led me to set before you, and no other way. God will not move in different ways in your life than he has recorded in the Scriptures. If you want to know how God will work for you, study your Bible -- that is how he will work with you. It is according to the gospel, and along these lines, and above all, "according to the preaching of Jesus Christ, because he is the one whose life, indwelling us, makes possible strength in our experience.

(2) Strengthening will be "according to the revelation of the mystery which was kept secret." What was the mystery? Simply the mystery that God would call out from Jew and Gentile alike the people who would belong to him and to each other. This is important because, if you are going to be strengthened in your Christian life, it means that it will be in connection with other believers as well. It is impossible for a Christian to grow strong all by himself. We need each other, and we need all the others who belong to the body of Christ. This is the great mystery -- as we share together in the life of our Lord Jesus and in each other, we grow strong in the Lord.
 

(3) strengthening is "according to the command of God to bring about obedience to the faith," i.e., the final great foundation upon which all our Christian growth and strength rests is the fact that God has set about a task which he will perform. As Paul writes to the Philippians, "being confident that he who has begun a good work in you will perform it until the day of Jesus Christ," {cf, notes Philippians 1:6}. (Ibid) 

Kept secret (4601) (sigao sigao from sige = silence) mean to be silent, to "hold one's peace", to keep in silence or  keep secret. The idea is to say nothing, keep still, keep silent (eg, Lu 9:36) or to stop speaking (eg, Lu 18:39). In the passive voice sigao means to be kept in silence. Finally, sigao can convey idea of keeping something from becoming known, to be concealed or to be kept secret (the only NT use with this meaning being here in Ro 16:25)

Sigao is in the perfect tense emphasizing the state of silence or kept in a state of silence

There are 10 uses of sigao in the Septuagint (LXX) (Exod. 14:14; Ps. 32:3; 39:2; 50:21; 83:1; 107:29; Eccl. 3:7; Isa. 32:5; Lam. 3:49; Amos 6:10) and here are some representative uses...

Exodus 14:14 "The LORD will fight for you while you keep silent."

Psalm 32:3 When I kept silent about my sin, my body wasted away through my groaning all day long. (Spurgeon's note)

Sigao is used 10 in the NT...

Luke 9:36 And when the voice had spoken, Jesus was found alone. And they kept silent, and reported to no one in those days any of the things which they had seen.

Luke 18:39 And those who led the way were sternly telling him to be quiet; but he kept crying out all the more, "Son of David, have mercy on me!"

Luke 20:26 And they were unable to catch Him in a saying in the presence of the people; and marveling at His answer, they became silent.

Acts 12:17 But motioning to them with his hand to be silent, he described to them how the Lord had led him out of the prison. And he said, "Report these things to James and the brethren." And he departed and went to another place.

Acts 15:12 And all the multitude kept silent, and they were listening to Barnabas and Paul as they were relating what signs and wonders God had done through them among the Gentiles. 13 And after they had stopped speaking, James answered, saying, "Brethren, listen to me.

Romans 16:25 Now to Him who is able to establish you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery which has been kept secret for long ages past,

1Corinthians 14:28 but if there is no interpreter, let him keep silent in the church; and let him speak to himself and to God....30 But if a revelation is made to another who is seated, let the first keep silent....34 Let the women keep silent in the churches; for they are not permitted to speak, but let them subject themselves, just as the Law also says.