In the doxology of Romans, Paul writes:
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 ESV)
The gospel is here spoken of as a mystery, which has been "kept secret for long ages, but has now been disclosed." This statement seems to imply that it was previously hidden, and only now made known through Christ.
At the same time, however, overlapping is the statement that this mystery "through the prophetic writings has been made known to all nations," which seems to imply that the mystery was already revealed back at the time of the prophets.
Even setting aside commitments to scriptural authority and biblical inerrancy, the mere fact that Paul adjoined these two statements gives one reason to think that this apparent contradiction is both intentional and non-contradictory. How does one reconcile these two statements?