Moroni and the Nephites had previously sworn an oath that they would fight when they essentially promised, “May we be trampled upon just as we are trampling on our coats, if we do not fight valiantly.” Now we learn in this verse (48:13) that Captain Moroni, who was “a man who was firm in the faith of Christ,” had himself sworn “to defend his people, his rights, and his country, and his religion, even to the loss of his blood.” This contrasts with an oath that Amalickiah had made: “He did curse God, and also Moroni, swearing with an oath that he would drink his blood” (Alma 49:27)! “On the other hand,” it says, “the people of Nephi did thank the Lord their God, because of his matchless power in delivering them from the hands of their enemies” (Alma 49:28).
When men are ordained to the Melchizedek Priesthood, it is with the making of an oath and a covenant. When men and women receive their endowments in the temple, they make covenants. That word covenant is just another word for an oath. There, we just say, “yes.” But what does Jesus say about swearing oaths? Don’t swear by the heavens or the hair of your heard; but “just let your speech be yea, yea, [or] nay, nay” (Matthew 5:37; 3 Nephi 12:37). When done right, that is how we are supposed to swear oaths: we say yes, and we mean it.