What does Romans 16:25 mean?

Paul concludes his longest biblical letter with a doxology, which is like a hymn or song of praise to God. Paul begins this doxology with the statement "to him who is able" followed by a long sentence describing what God has done and is doing in and through the gospel of Jesus Christ. Not until the end of the sentence will Paul finish the thought by declaring the glory of God.

First, Paul reminds the Christians in Rome that God can strengthen them by or through "my" gospel and the preaching of Jesus Christ. Paul insists that "his" gospel, meaning the one he has consistently preached and has presented in this letter to the Romans, is the final word on what is true about Jesus. Nobody should accept any other version of the gospel of Jesus. That would be a false gospel (Galatians 1:8–9).

The true gospel, the true "good news," is that through faith in Jesus, who died to pay the penalty for our sins, all human beings can be forgiven and receive a righteousness from God that cannot be attained in any other way (Romans 3:23; 6:23, 10:9–12). This is what it means to be saved, to be given a place in God's family forever where we will share in His glory.

Paul describes this great truth as a mystery. It was not something discovered or uncovered by himself or anyone else. Instead, it was revealed by God when the time was right. Until then, it remained hidden for the "long ages" as generations of humanity came and went until the arrival of Jesus Christ, the Son of God in human form.