God our Father
The name of God is an important element in showing who God is and it is the main way by which God is known in the Old Testament, but it is not the way in which followers of Jesus Christ would address God.
Those who are in Christ are given the honour of referring to God as their Father:
"After this manner therefore pray ye: Our Father which art in heaven, Hallowed be thy name." (Matthew 6:9)
This new way of referring to God is a privilege which shows the connection between the follower of Jesus and God. When we are baptized we become adopted children of God and we can then call God our father.
"For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, 'Abba! Father!' The spirit himself bears witness with our spirit that we are children of God, and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him." (Romans 8:1517)
The word 'Abba' is an Aramaic word which means 'Father' - it is a term of affection rather than the more formal word 'Ab'.
This passage refers to our calling God our Father because we have received the 'Spirit of adoption'
This, of course, is simply another way of saying that we have been adopted by God as children; the spirit of adoption is merely a figure of speech, just as the spirit of slavery is a figure of speech which refers to our being prisoners of sin. It is the knowledge that we are children of God which bears witness to us.
The passage tells us that we can receive the assurance that we are children of God because this spirit of adoption tells us so.
Because of this we can call God our father.