We can invoke God as "Father" because he is revealed to us by his Son become man and because his Spirit makes him known to us. The personal relation of the Son to the Father is something that man cannot conceive of nor the angelic powers even dimly see: and yet, the Spirit of the Son grants a participation in that very relation to us who believe that Jesus is the Christ and that we are born of God.
Source: Catechism of the Catholic Church (Vatican.va)