The disciples of Jesus who knew the true God were the three who were blessed to remain on the earth until Christ came again. Their mission was to bring souls unto Christ (see 3 Nephi 28:4–10). Thus their purpose in ministering to Mormon and Moroni was undoubtedly in this capacity. They had either ministered to them earlier, and Moroni is recollecting their visit, or they had ministered to them in regards to their keeping the records. The three disciples may have been among the lost tribes since they had departed from the Nephites because of the wickedness of the people (Mormon 8:9; see Mormon 1:13). We do not know the degree of righteousness among the lost tribes at this time, but apostasy was rampant among the rest of the tribes of Israel, and there was no work of the Lord going on among other non-Israelites that we know of.