“This is how you are to pray,” Christ said: “Our Father in heaven.” This new person, born again, restored to God by grace, says first of all “Father” because this one has now become an heir. So any who have believed in his name and have become children of God ought now to begin to offer thanks and to declare themselves God’s children, when they speak of God as their Father in heaven.

How indulgent it is of the Lord, what exuberance of condescension and goodness toward us, to permit us when praying in God’s presence to address ourselves to God as Father—a name which none of us would have dared to reach in prayer, had he himself [Christ] not allowed us to pray. We should therefore recollect and feel  that, when we call God a Father, we ought to act like children of God, and if it comforts us to regard him as our Father, let us so act that he may be comforted in us. Let us conduct ourselves as temples of God, and God will remain in us.

—Cyprian, Bishop and Martyr of Carthage, On the Lord’s Prayer