In the Old Testament it was an ideal, a dream, an aspiration, to have God be the father to a generation. Paul seems to quote the Old Testament when he writes “I will be a father to you, and you shall be sons and daughters to me, says the Lord Almighty” (1 Cor 6:18) but you will not find that anywhere in the Old Testament. Some Old Testament passages exist where God foresees a future in which He would have a people and He would be calling the people His sons and daughters, e.g. Isaiah 43:6, Hosea 1:10, Jeremiah 3:19 much like when he freed them from Egypt referring to the Israelite people as “My firstborn son” Exodus 4::22. For God to be the Father, there would need to be a restored people to be sons and daughters to God. The Messiah would bring this about, such was the hope, the dream, the ideal, the aspiration. The Messiah would be the true Son who would bring about a nation of sons and daughters. Jesus came referring to himself consistently, constantly, as the Son and speaking of a Father as his own father and as the Father, directing his hearers to believe and grasp this. In hearing him, believing in him as this Son, holding to his teachings as those from his Father, they could become sons and daughters of this Father. He occasionally revealed it that by this Father he meant the Old Testament God of Abraham, God of Isaac, God of Jacob (for example Matthew 22:32), although he seems to have carefully avoided making this connection explicit. It is in writings of the apostles, especially Paul, that we find this connection made explicitly. Paul consistently referred to the Father alone as God (perhaps only one exception in Romans 9). He clearly wrote about God, the God in the Old Testament, as the Father of whom Jesus had spoken. So it is in belonging to Jesus that people become a people with God as their Father: God the God of Abraham, but the God of the Gentiles too.
Romans 4:3 quoting Genesis 15:6: "Abraham believed God, and it was counted to him as righteousness."
Romans 3:29-30: "Is God the God of Jews only? Is he not the God of Gentiles also? Yes, of Gentiles also, since God is one."
In the sayings of Jesus, by becoming a people with his Father as their Father, the Father is then this people’s true God. Simply calling Him their God is not enough. They must become true children of this Father for Him to be their God. Not lying. Not hating. Loving Jesus and loving his Father. If they loved this Father they would love His Son too. Then loving each other, being loved by this Son. It is all through Jesus, the Messiah.
