One God and Father - A Christian knows God as Father
There is ... one God and Father of all, who is above all, and through all, and in you all. Ephesians 4:6
Only one in Christ can call God, Father - the believer can because he has been adopted as a son or daughter.
For you did not receive the spirit of bondage again to fear, but you received the Spirit of adoption by whom we cry out, "Abba, Father." Romans 8:15
And because you are sons, God has sent forth the Spirit of His Son into your hearts, crying out, "Abba, Father!" Therefore you are no longer a slave but a son, and if a son, then an heir of God through Christ. Galatians 4:6-7