Paul's great letter to the Romans ends with a doxology -- a moment of praise.
(There is widespread scholarly opinion that Paul did not write these verses; however, the person who did and the editor who placed them near the end of the letter [commentaries cite the various placements of the doxology] clearly presumed that these words reflected Paul's views). Earlier in Romans, Paul has stated that the fundamental human act in relation to God should be acknowledgement of God (Romans 1:28), and so the giving of honor and thanksgiving to God (Romans 1:21). Humanity's ability to be human is built on the primary action of recognizing God and so of honoring and thanking God. The essential human stance is a focus on God and a profound awareness that God is, as a famous theologian once said (Tillich), 'the ground of being'. Consequently, the ideal human disposition is praise of God.
The perfect mode of being human is to live for God's glory. Paul (or an heir of Paul) models this at the end of Romans: "to the only wise God be glory for evermore through Jesus Christ! Amen" (16:27).
Praise of God is an ideal because humanity, in Paul's view, has been drawn away from its natural state. Sin, and humanity's cooperation with Sin, has distorted humanity so that, typically, human beings look to themselves and look after themselves as if God did not exist. The praise of God is foreign; what is normal is the praise of humanity. This means that what should be natural has become an ideal.
The good news, however, is that this ideal is now an offer. Humans can now live in praise of God because they can now live in the "obedience of faith" (Romans 16:26; cf. Romans 1:5). God has made available, through the gospel, the capacity to live apart from the dominion of Sin; to live in Christ (Romans 8:1-2), which is to live in the obedience of faith, as Christ did (Philippians 2:8) -- the result of which is the glory of God (Philippians 2:11).
This way of faithfulness to God, of obedience to God, of praise of God is available because God has taken the initiative and revealed the mystery (16:25). Paul thinks of the good news as the apocalyptic revelation of God's Son (see Galatians 1:16; and look at the commentary on Galatians by J. L. Martyn). It is not that the gospel itself is mysterious. It is that the gospel is the unveiling of what had until now been a mystery.
Two things to note here: First, the gospel is an unveiling of something that was in existence. The gospel is, then, not new in the divine scheme of things. It has been in God's universe, as the words of this passage say, "for long ages" (16:25). This is why the "prophetic writings" (16:26) are one of the vehicles through which the mystery is revealed. Even though the scriptures of the synagogue are prior to the "preaching of Jesus Christ" (16:25), they reveal that preaching. This is the case because the kerygma of Jesus Christ is only new from the human point of view.
Other writings of Paul will take up this feature of the wonder of the gospel. Colossians, for instance, will speak of Christ's activity in the creation of the world (Colossians 1:16) and of how all things hold together in Christ (Colossians 1:17). The event of the cross is placed in a context much broader than the historical life of Jesus. The event of the cross is seen as an episode in a narrative that spans back to the creation of the world and forward to the final defeat of everything that opposes God's holiness.
The second thing to note is that the mystery is a mystery only until God decides to unveil it. Now there is no mystery in regards to the good news and the preaching of Jesus Christ. God has revealed the mystery.
Believers, then, stand in the light of revelation. This is all the more reason to praise God. God's character as the one who cares enough to help God's creatures live in their most perfect way, in the way of obedience of faith, is seen more fully now. The good news reveals God's commitment to helping humanity live apart from Sin and so apart from disobedience; to live as we were created to live -- towards God and not towards ourselves.
This passage places the incarnation, which we will shortly celebrate, in the broad arena of God's never ending, always existent desire for humanity to live in peace. The reconciliation that is offered in the gospel is the reconciliation to what humanity was created to be. The goal of reconciliation has always been at the heart of God's cosmos. Now it has been revealed. Believers in Christ are privy to the revelation of the mystery which is the good news -- that Jesus Christ came into the world, died for our sins, was raised and that he will return to complete the defeat of Sin.