Ancient Context: In Abinadi’s discourse, this is the logical follow up to his assertion that God himself would come among man. This particular discourse is necessitated by the title “Son of God.” As we saw with the conflict between Sherem and Jacob, the understanding of this atoning Messiah was a Nephite revelation, one not required by the brass plates text (neither Sherem’s interpretation nor that of ancient Israel required them to clearly understand the atoning Messiah in the clear way that the Nephites did).
While we don’t know the sources available to Zeniff and his descendants, we may suppose that this understanding of the atoning Messiah as the Son of God was part of Nephite religious understanding (Nephi specifically notes this atoning Messiah as the Son of God: see 1 Ne. 10:17; 11:7, 11:18, 11:24; 2 Ne. 25:16 - while these citations are on the small plates, one would hardly expect Nephi not to teach this principle openly).
Thus the reason for this explanation of the relationship of the Father and Son in Abinadi is created precisely because there might be a reason to think them separate, and that would undermine his contention that God himself was going to come among men. In other words, it is confusing to modern readers for the very reason Abinadi brought it up in the first place.
Abinadi uses the Son of God as a title, and in so doing, also uses Father as a title. With both of those functions removed from strict biology and functioning in the symbolic realm, he can reassign symbolic attributes.
What is interesting is that the atoning Messiah becomes the Father because he is begotten of God, a definition we would typically apply to the reason for becoming the Son. Of course we understand God the Father as the literal parent of Jesus, but Abinadi’s argument is different. He is using Jehovah (premortal Jesus) as the Father, and Jesus (the mortal) as the Son. Thus his distinction is based on the realm of operation more than the biology. Father is in heaven, son is on earth.
Modern Context: Daniel Ludlow provides the following explanation of how this verse may be understood in the light of our modern understanding of the nature of the godhead:
“Jesus Christ is referred to several times in the Book of Mormon as both the Father and the Son. (Mosiah 15:1-4; Ether 3:14.) The question might well be asked: In what way (or in what sense) is Jesus Christ both the ”Father“ and the ”Son"? The words Father and Son are titles rather than names; thus they may be used to refer to more than one person. The term Father may rightfully be used to refer to Jesus Christ in the following areas:
The term Son also has varied meanings. Jesus Christ is rightfully referred to as the Son in the following senses: (1) Jesus Christ is the firstborn of God in the spirit (Colossians 1:15-19; D&C 93:21); (2) Jesus Christ is the Only Begotten Son of God in the flesh (Jacob 4:5, 11; Alma 12:33-34; 13:5; John 1:18, 3:16); (3) Jesus Christ submitted his will to the will of his Father (Mosiah 15:2-7). (Ludlow, Daniel H. A Companion to Your Study of the Book of Mormon. Deseret Book, 1976, pp. 183-4).