“which he promised beforehand through his prophets in the holy Scriptures,” -Romans 1:2
In the introduction of his letter to the Christians in Rome, Paul establishes his office as one who has been entrusted with the gospel, the good news for which the children of Israel had been waiting. And it was not just for them, but for the whole world.
The gospel is the good news that what had been promised by God’s prophets in the holy Scriptures (i.e., Old Testament) concerning Messiah had now been realized in the person of Jesus Christ, as he will demonstrate in the verses following.
Paul preaches a similar message in Antioch of Pisidia when he says, “And we bring you the good news that what God promised to the fathers, this he has fulfilled to us their children by raising Jesus, as also it is written in the second Psalm, “ ‘You are my Son, today I have begotten you.’” -Acts 13:32–33
At the close of this letter to the Romans, Paul will exhort his readers with an application of what he said here at the opening of the letter, namely that what was promised in ages past is being fulfilled now in his time.
“Now to him who is able to strengthen you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery that was kept secret for long ages but has now been disclosed and through the prophetic writings has been made known to all nations, according to the command of the eternal God, to bring about the obedience of faith— to the only wise God be glory forevermore through Jesus Christ! Amen.” -Romans 16:25–27
What Paul establishes here is that God keeps his promises and can be trusted. Moreover, what has been manifested in Christ Jesus, this fulfillment of the promise, is the most significant turning point in human history so far—the redemption of the cosmos and the creation of a new humanity has begun.
From this time forth, nothing will ever be the same.