Had the revelations given to Paul been kept secret until his time?

Yes. Paul's revelations had indeed remained hidden until God, in His sovereign grace, revealed them during Paul's lifetime. The Old Testament and the Gospels knew nothing of the Gospel of the grace of God, but Paul repeatedly announced a mystery now made known.

Les explains from Deuteronomy 29:29 that the secret things belong to the Lord, while what is revealed belongs to us and our children. Moses wrote that to Israel, but Les says the same principle applies today: God keeps things secret until, in His sovereign grace, He sees fit to release and reveal them. This is the Pauline doctrine—it was secret. The Old Testament knew nothing of the Gospel of the grace of God, and Matthew, Mark, Luke, and John knew nothing of Paul's Gospel that went to Jew and Gentile alike. Paul repeatedly says behold I show you a secret or mystery, and in 1 Corinthians 2 he refers to the wisdom God gave the apostle, revelations kept secret and now coming into full view. He speaks the wisdom of God that has come out of a secret.

1 Corinthians 2:6

Howbeit we speak wisdom among them that are perfect: yet not the wisdom of this world, nor of the princes of this world, that come to nought:

What does 1 Corinthians 2:7 mean by the wisdom of God that has come out of a secret?

The wisdom of God that has come out of a secret is the previously hidden divine revelation—the Gospel of the grace of God—kept secret in Old Testament times and in the Gospel accounts, then revealed through Paul. It is not the wisdom of the world, but God’s sovereignly unveiled truth for Jew and Gentile alike.