Why does the Old Testament focus on the Hypostasis of the Father?

The Old Testament emphasizes the love and compassion of God the Father for those who fear Him. It says, “For as the heavens are high above the earth, so great is His mercy towards those who fear Him … As a father pities his children, so the Lord pities those who fear Him” (Ps. 103:11, 13). God is described as a father to the people of Israel (Deut. 32:6; Mal. 2:10; Isa. 63:16; 64:8); not because He neglects the gentiles, but because he was preparing the whole world to receive the incarnate Word of God, the Savior of the world.

We cannot deny that some gentiles were concerned for the salvation of their souls, such as Ruth the Moabite and Rahab the Canaanite harlot; both of whom appear in the genealogy of the Lord Christ (Matt. 1:5). The former is a symbol of gentile acceptance of faith in the Savior; and the latter is a symbol of opening the door to the salvation of sinners who come return to the Lord, believe in the Savior, and become members in the Church.

As for the New Testament, God the Father proclaims that He is the Father of all those who accept Him and believe in Him, without distinction between Jews and other peoples. As they return to their Father, they hear His voice, saying, “for this my son was dead and is alive again; he was lost and is found” (Luke 15:24). Thus, the New Testament reveals the mystery of the Holy Trinity to the Jews as well as to the Gentiles.