Did the Old Testament reveal that Gentiles would be brought into a relationship with God?
The Old Testament did not reveal that Gentiles would be brought into a relationship with God. That truth was kept secret as part of the mysteries, only revealed later in the Age of Grace through Paul's gospel. There was no concept of Gentile inclusion in the prophetic program.
Les explains that prophecy had run its course, Christ came according to prophecy, was rejected, and crucified. With Israel's continued rejection, the prophetic program of Jew only came to a screeching halt. God's time clock stopped, and He opened a whole new vista of Scripture: the mysteries. In Romans 16:25 Paul writes about the revelation of the mystery, the preaching of Jesus Christ according to his gospel. For almost two thousand years we have not been in the prophetic program but in the Age of Grace. These things were kept secret in the Old Testament; there was no concept then of Gentiles being brought into a relationship with the Almighty God.