God had a relationship with great man in the bible such as Abraham, Jacob, Isaac, Enoch, Moses, Joshua, David etc but none of them did He call Son nor did they know Him as Father. Jesus was the only begotten Son of God, until became redeemed by the blood and born of the spirit. Now we are sons of God begotten of the Holy Spirit we are privileged to able to call Him father.
We now have a relationship with God, that no man under the Old testament ever had. The angels too envy us. We need to understand what it means for God to be our Father so that we can reap its benefits in full!