On one level, God is our father because he created us. For example, in Luke’s genealogy of Jesus, he says:
Luke 3:38 the son of Enos, the son of Seth, the son of Adam, the son of God.
And Deuteronomy 32:6
Do you thus repay the LORD, you foolish and senseless people? Is not he your father, who created you, who made you and established you?
Isa 64:8
But now, O LORD, you are our Father; we are the clay, and you are our potter; we are all the work of your hand.
So God is our father on a purely physical level. But in a much greater way, we can become sons of God – by God calling us to be sons (John 6:44) and us responding to God’s call. We have to be born of God and change from our natural fleshly life to a life in the spirit. As Jesus said to Nicodemus:
John 3:5-8 “Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. (6) That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. (7) Do not marvel that I said to you, ‘You must be born again.’ (8) The wind blows where it wishes, and you hear its sound, but you do not know where it comes from or where it goes. So it is with everyone who is born of the Spirit.”
We have to become like God – our mind has to become spiritual not carnal. We have to look to Jesus and follow him. We have to believe and be baptised into Jesus. Galatians 3:26-27 explains this:
for in Christ Jesus you are all sons of God, through faith. For as many of you as were baptized into Christ have put on Christ.
We take up our cross daily as Jesus told us (Matt 16:24) – we crucify the lust of the flesh, the lust of the eyes and the pride of life. We no longer serve sin, but God. 1 John 3:9-10:
No one born of God makes a practice of sinning, for God’s seed abides in him, and he cannot keep on sinning because he has been born of God. (10) By this it is evident who are the children of God, and who are the children of the devil: whoever does not practice righteousness is not of God, nor is the one who does not love his brother.
So God is our father if we do these things, and the blood of Jesus cleanses us from all sin.
1 John 5:1-5 Everyone who believes that Jesus is the Christ has been born of God, and everyone who loves the Father loves whoever has been born of him. (2) By this we know that we love the children of God, when we love God and obey his commandments. (3) For this is the love of God, that we keep his commandments. And his commandments are not burdensome. (4) For everyone who has been born of God overcomes the world. And this is the victory that has overcome the world–our faith. (5) Who is it that overcomes the world except the one who believes that Jesus is the Son of God?