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:
The Old Testament proclaimed the Father openly, and the Son more obscurely. The New manifested the Son, and suggested the Deity of the Spirit. Now the Spirit Himself dwells among us, and supplies us with a clearer demonstration of Himself. For it was not safe, when the Godhead of the Father was not yet acknowledged, plainly to proclaim the Son; nor when that of the Son was not yet received to burden us further (if I may use so bold an expression) with the Holy Ghost; lest perhaps people might, like men loaded with food beyond their strength, and presenting eyes as yet too weak to bear it to the sun’s light, risk the loss even of that which was within the reach of their powers; but that by gradual additions, and, as David says, Goings up, and advances and progress from glory to glory, the Light of the Trinity might shine upon the more illuminated. For this reason it was, I think, that He gradually came to dwell in the Disciples, measuring Himself out to them according to their capacity to receive Him, at the beginning of the Gospel, after the Passion, after the Ascension, making perfect their powers, being breathed upon them, and appearing in fiery tongues. (Theo. Ora. V, XXVI)
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.