Translation: Mormon’s description of the oaths (v. 10) that they had “sworn by all that had been forbidden them by our Lord” ties the language of the oath to that of the Messiah’s Sermon at the Temple. (See commentary accompanying 3 Nephi 12:34.)
This verse shows a weak point in the textual transmission. Mormon’s reference to 3 Nephi is imprecise, and he has incorrectly turned an explanation into a prohibited oath. Matthew 5:34 states: “But I say unto you, Swear not at all; neither by heaven; for it is God’s throne.” The version in 3 Nephi is virtually identical for the first part and exactly identical in the section dealing with the prohibited oath: “But verily, verily, I say unto you, swear not at all; neither by heaven, for it is God’s throne” (3 Ne. 12:34). Both verses agree that swearing by heaven is prohibited and give the reason: heaven is God’s throne and therefore sacred. In contrast, Mormon 3:10, states: “And they did swear by the heavens, and also by the throne of God, that they would go up to battle against their enemies, and would cut them off from the face of the land.” In other words, the reason originally given for prohibiting the oath has itself become the prohibited oath. The model is clear, but Mormon’s reference to it is inaccurate. I hypothesize that the error lies in the translation process. As Joseph Smith translated the material before him, he understood that some oaths were prohibited, but because he was not reading those oaths from either the Bible or 3 Nephi text, he mistakenly paraphrased the reason as the oath.