In Isaiah 52:7, Abinadi switched out "Thy God reigneth" with "The Son reigneth" (Mosiah 15:20), and thereby emphasized that the Son is the Messiah who will come, will redeem, will reign, and will do the things that Isaiah prophesied about in chapters 52 and 53. Having focused on the Son, Abinadi needed to explain in what sense Christ is the Son and also the Father. King Benjamin made a similar statement when he declared, "And he shall be called Jesus Christ, the Son of God, the Father of heaven and earth, the Creator of all things from the beginning" (Mosiah 3:8).
Christ will be the Son because He will come down be born to a human mother in a miraculous way. However, he will also be the Father, because he is the creator of heaven and earth (Mosiah 3:8), and because we are spiritually born again as Christ’s sons and daughters (Mosiah 5:7).
Abinadi explained this concept by highlighting aspects of the spirit and the flesh when talking in terms of the Father and the Son. If you replace the word son with the word flesh and the word Father with the word spirit in these passages, things start to become clearer in what Abinadi was saying. In Mosiah 15:1 Abinadi said, "I would that ye should understand that God himself shall come down among the children of men." This refers to Mosiah 14:2 (Isaiah 53:2) passage that he recited: "He shall grow up before him as a tender plant." The "he" in this passage is the Son. The "him" is God the Father. One God will come among the children of men and shall redeem his people.
Then in Mosiah 15:2 Abinadi explains, "Because he dwelleth in the flesh, he shall be called the Son of God." Because of the flesh, he is the Son, but because he has made the flesh subject to the will of the Father, he will be the Father and the Son. "The Father because he was conceived by the power of God" (15:3), so he is spiritual, and the Son in the sense that "he is of the flesh" (15:3), that is because he will condescend and come as a mortal.
In Mosiah 15:4, The Fatherhood refers to Christ’s immortality and his divine origin with the Father and his oneness with the Father. "They," meaning both functions or attributes constitute one, premier God. The word "heaven" means the sky. Thus, Christ is the very Eternal Father of Heaven and Earth, but not of the whole cosmos. That is God the Father.
Mosiah 15:5 reads, "and thus the flesh becomes subject to the spirit." In other words, the Son becomes subject to the will of the Father, being that one God who suffers temptations, suffers mockery, and is disowned by his people. And after mighty miracles, even as Isaiah says, he shall be taken "as a sheep before the shearer." He remains dumb, and he will die for us.
Abinadi had to start here because the priests of Noah likely had a hard time understanding how God could come down and leave the heavens empty. This would be the basis for the blasphemy charge brought against Abinadi, that he had said that Christ would come down, and thus that God would somehow abandon his throne, the leadership, and rule of the world. That would certainly be considered unbecoming of God who is the King of Heaven. We can see how Abinadi was trying to lead the priests of Noah to see a little more clearly how Christ could still be God and the Father could still be in heaven.
Abinadi also had to be careful to be sure that he said that "they are one," so that he did not contradict the first of the Ten Commandments, "Thou shalt have no other god before me." That is usually where monotheism is thought to stem from, but in fact, there are two divine beings already mentioned in Isaiah 53:2 ("he shall grow up before him") and 53:10 ("Yet it pleased the Lord to bruise him"). When you read Abinadi’s explanation of that, along with what Isaiah had said, and further with our understanding of the Godhead in mind, all of this becomes very clear.
Book of Mormon Central, "How is Christ Both the Father and the Son? (Mosiah 15:2)," KnoWhy 92 (May 4, 2016).
Paul Y. Hoskisson, "The Fatherhood of Christ and the Atonement," Religious Educator 1, no. 1 (2000): 71–80.
Jared T. Parker, "Abinadi on the Father and the Son: Interpretation and Application," in Living the Book of Mormon: Abiding by Its Precepts, ed. Gaye Strathearn and Charles Swift (Salt Lake City and Provo, UT: Deseret Book and Religious Studies Center, Brigham Young University, 2007), 136–50.