1. Mormonism is not “anti-Trinitarian”. Mormonism believes 100% in the Trinity—the Father, the Son and the Holy Ghost. The LDS first article of Faith states:
We believe in God, the Eternal Father, and in His Son, Jesus Christ, and in the Holy Ghost.What it does not believe in is the Trinity of post-Apostate Christendom—the “three in one and one in three” idea. Mormonism believes in the Trinity of the Bible, which consists of three separate beings united in mind, purpose, and will.
2. Mosiah 15:1–7 does not teach the Trinity of post-Apostate Christendom. The doctrine that it teaches is vastly different. If the passage is read carefully, that soon becomes apparent. Furthermore, the same doctrine, expressed in different words, is taught also in D&C 93:1–4. (Read the complete section for more comprehensive insight.) So there has been no “change” in LDS doctrine of the Godhead between the Book of Mormon and the D&C. The doctrine remains the same.
3. The doctrine of the unity of God, or of the members of the Trinity, as it is found in the Book of Mormon, is equally as much taught in the Doctrine and Covenants and the Pearl of Great Price. D&C 20:17, 27, 28; 68:8; Moses 1:6, 24; 5:9; 6:66 are some good examples. Thus the idea that doctrine has “changed” between the Book of Mormon and the Doctrine and Covenants, or the Pearl of Great Price, is not true. (If these folks were to be believed, one would think that LDS didn’t believe in the Book of Mormon any more!)
4. At the same time, the doctrine of the separate identity of the members of the Godhead is equally taught in the Book of Mormon, as it is in the D&C and the Pearl of Great Price. 2 Nephi 31:11, 12, 14, 15, 18; 3 Nephi 11:6–8 are some examples. Indeed, Joseph Smith received his First Vision, in which the doctrine of the separate identity of the members of the Godhead was revealed to him for the first time (more comprehensively in fact than to any previous prophet), long before he had translated the Book of Mormon or organized the Church.
5. LDS doctrine has not changed and does not change. It has increased and progressed, which is a different thing. God teaches mankind a little bit at a time, not all at once. Knowledge comes “line upon line, and precept upon precept”. God continually revealed to Joseph Smith new truths which He had not revealed to him at an earlier period; but that did not mean that doctrine had “changed”. You may learn something in tenth grade which appears to contradict what you learned in first grade. That is not because “truth” has changed, but because you know more. There is nothing revealed in the Book of Mormon that actually contradicts what was subsequently revealed in the D&C or the Pearl of Great Price. But God has revealed additional truths as time went by.
6. As far as the Bible is concerned, it is just as easy to go through it, and to compile a comprehensive exegesis supporting the doctrine of the plurality of Gods, or of the Trinity, as it to compile one supporting their unity.