You're getting to a VERY large topic here. Paul makes clear many, many times that he received NEW information that was never before revealed.
Rom 16:25-27 KJV Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began, (26) But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith: (27) To God only wise, be glory through Jesus Christ for ever. Amen.
Note that Paul is saying that the revelation of the mystery was keep secret since the world began, but is NOW made manifest.
The "and by the scriptures of the prophets" refers to the "power to stablish", which requires knowledge of BOTH the old and the new.
Paul also says that this mystery was "hid in God".
Eph 3:9 KJV And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God,who created all things by Jesus Christ:
... Until it was revealed to and through Paul.