Was the secret of Christ known in Old Testament times?
No. The mystery, or secret, of Christ was not made known to the sons of men in other ages; it remained hidden until God revealed it by revelation to the apostle Paul and to the holy apostles and prophets. Paul wrote that this dispensation of grace was given to him for the Gentiles.
Les explains that in Ephesians chapter 3 Paul writes to predominantly Gentile churches in Ephesus as the prisoner of Jesus Christ for the Gentiles. Just as God gave the law to Moses for Israel, He gave the doctrines of grace to Paul, and Paul took them to the Gentiles. Paul received the mystery, the secret of Christ, by revelation, not by going to a seminary or theologian. Verse 5 says this secret in other ages was not made known to the sons of men as it is now revealed to the holy apostles and prophets. Others like Barnabas and Silas followed Paul.
Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit;