These instructions are being given because of disputations. We do not know what those disputations might have been. The evidence from the text suggests that this much of the instruction was certainly understood. The Savior indicates here that there are two aspects to the baptism that must occur. The first is a prior repentance, and the second that they “go down and stand in the water.” Both of these aspects of baptism are present in the Nephite understanding of baptism.
As an example of the need for repentance prior to baptism we have 2 Nephi 31:11-15 and Alma 7:13-16. The best example of the importance of “going down in” to the water comes from Alma’s baptisms at the waters of Mormon:
Mosiah 18:12-16
12 And now it came to pass that Alma took Helam, he being one of the first, and went and stood forth in the water, and cried, saying: O Lord, pour out thy Spirit upon thy servant, that he may do this work with holiness of heart.
13 And when he had said these words, the Spirit of the Lord was upon him, and he said: Helam, I baptize thee, having authority from the Almighty God, as a testimony that ye have entered into a covenant to serve him until you are dead as to the mortal body; and may the Spirit of the Lord be poured out upon you; and may he grant unto you eternal life, through the redemption of Christ, whom he has prepared from the foundation of the world.
14 And after Alma had said these words, both Alma and Helam were buried in the water; and they arose and came forth out of the water rejoicing, being filled with the Spirit.
15 And again, Alma took another, and went forth a second time into the water, and baptized him according to the first, only he did not bury himself again in the water.
16 And after this manner he did baptize every one that went forth to the place of Mormon; and they were in number about two hundred and four souls; yea, and they were baptized in the waters of Mormon, and were filled with the grace of God.
There must have been some disputations with those who leaned to the beliefs of perhaps the Nehors. The true Nephite record holds the same information as given by the Savior at this time.