“The Father has a unique relation to the incarnate Son within the being of God. God’s revelation as the Father does not refer to his general fatherhood with respect to all his creatures. Moreover, as Toon comments, the name Father is not merely a simile (as if God is simply like a father) or even a metaphor (an unusual use of language drawing attention to aspects of God’s nature in surprising terms), but rather a definite personal name. In contrast, the term mother, when used in reference to God in the OT, is a simile, but never a metaphor, and it is completely absent in the NT. Father is the proper name of God and does not merely describe what he is like” (Robert Letham, The Holy Trinity, 36).
“‘Abba’ [Father] was used by the early Christians in prayer. . . . This custom stems from Jesus’ usual way of addressing God. His followers adopt it because they believe that they share his own natural relation to the Father” (36).