Paul speaks about the fullness of times (I think, in Romans), upon the arrival of which the grace was manifested in the Son of God, but I wonder why was it needed for the Father to conceal the fact that He had the Son? I mean, the fact that God would've saved humanity through the sacrifice of His own Son might've really needed to be concealed for people to realize through their own history that they were simply not able to match the standards of righteousness needed for eternal life, but why was it needed to hide away from humans the fact the God had the Son?
This is to some extent the idea of progressive revelation, which is a concept that originated in Origen and was fully expounded in Gregory the Theologian's Fifth Theological Oration (which was in defence of the divinity of the Holy Spirit against the Pneumatomachai).
This is the purple passage:
So the idea that Gregory expounds (which is widely accepted) is that you get the revelation in stages, not all in one go. The doctrine of the Trinity (which for Gregory is the zenith of the Christian faith) would be meaningless outside the history of Israel in which the world had come to know God.
There are hints in the Old Testament, e.g. in the messianic prophecies of Proto-Isaiah, the Servant Songs of Deutero-Isaiah, various psalms (especially the royal ones), perhaps even in certain narrative passages (e.g. Jacob wrestling with God); these hints are revealed fully in the New Testament. Likewise, the New Testament has a foreshadowing of the revelation of the Spirit in the Church, but not a full acknowledgement of the full divinity of the Spirit.
You might also realise that there is a kind of progressive revelation in the NT of the divinity of Christ. If we take the Gospel of Mark, for instance, the knowledge of who Jesus is is a "secret" all the way through, with hints and foreshadowings; it is only on the Cross that he is revealed publicly. (Mark 15.39)
Because that is the whole point of the New Testament. How to have faith that Jesus is God. Israel thought the Messiah was going to be the Son of David (Mark 12:37). But the Gospels prove that Jesus is LORD. The Son of God. And we do have Isaiah 9:6 Jesus, the prince of peace.