There are a number of examples where God does not reveal his purpose until later. The biggest example that stands out is the Mystery of Christ.
Ephesians 3:3-6 (KJV)
3 How that by revelation he made known unto me the mystery; (as I wrote afore in few words,
4 Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)
5 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;
6 That the Gentiles should be fellow-heirs, and of the same body, and partakers of his promise in Christ by the gospel:
Romans 16:25 (KJV)
25 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,
Paul explains the Gospel simply in 1 Corinthians:
1 Corinthians 15:3-4 (KJV)
3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
4 And that he was buried, and that he rose again the third day according to the scriptures:
Why wasn’t this revealed prior to Paul and instead to perhaps John the Baptist?
We have to understand that we’re in a cosmic battle where events happen such as a chess match. God makes a move, then Satan tries to counter that move. God makes a perfect creation, Satan deceives Eve to eat the forbidden fruit and Adam follows which brings sin into the world. God pronounces a war with Satan in Genesis 3:15, Satan tries to corrupt all DNA with fallen angels producing hybrids. God wipes out everything with a global flood.
These moves go back and forth until God’s plan becomes clearer. The Messiah is to come through Abraham, Isaac and Jacob to the nation of Israel through a very specific genealogy ending in Jesus. Many attacks narrow to focus on wiping out Israel with nephelim to be destroyed by Joshua to Haman wanting to kill the Jews, to idolatry and so on. Satan sees God’s move, then makes an attack.
When Satan understands that Jesus is the Messiah, he wants Jesus killed. Jesus counters by orchestrating all events to the cross to fulfil numerous specific levetical and prophetic specifics. I’m sure he rejoiced to see Jesus die on the cross, but didn’t suspect that Jesus would rise again. He didn’t understand that the purpose for Jesus dying was to pay the price for everybody’s sin. Jesus won the chess match. This is why this was a mystery before the foundation.
When you’re in a battle, you don’t give away all of your moves.