One of the unique aspects of Christianity is that God is called "Father." Many other religions view God as a "master," or as a distant being that is not even named.
However, Christians call God "Father," and we believe that it is not something that humans invented or created.
The Catechism of the Catholic Church makes this clear in its section on the Our Father:
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.
The Incarnation
Jesus is the one who taught us to pray "Our Father," inviting us into a filial relationship with God:
Jesus revealed that God is Father in an unheard-of sense: he is Father not only in being Creator; he is eternally Father in relation to his only Son, who is eternally Son only in relation to his Father: “No one knows the Son except the Father, and no one knows the Father except the Son and any one to whom the Son chooses to reveal him.
St. Thomas Aquinas also writes about this reality in his Summa Theologiae:
The proper name of any person signifies that whereby the person is distinguished from all other persons. For as body and soul belong to the nature of man, so to the concept of this particular man belong this particular soul and this particular body; and by these is this particular man distinguished from all other men. Now it is paternity which distinguishes the person of the Father from all other persons. Hence this name "Father," whereby paternity is signified, is the proper name of the person of the Father.
God the Father is not man or woman
One important thing to keep in mind is that God the Father is not man or woman. He is pure spirit and transcends human distinction.
The Catechism points out this important point as well:
We ought therefore to recall that God transcends the human distinction between the sexes. He is neither man nor woman: he is God. He also transcends human fatherhood and motherhood, although he is their origin and standard: no one is father as God is Father.
On the other hand, God the Son became a man and Jesus has a male body.
The main point is that calling God "Father" is not something we created, but was something revealed to us.
