3 Nephi 8:1

And now it came to pass that according to our record, and we know our record to be true: for behold, it was a just man which did keep the record; for he truly did many miracles in the name of Jesus; and there was not any man which could do a miracle in the name of Jesus, save he were cleansed every whit from his iniquity.

Our records are true. A good man kept them. He did many miracles in Jesus’ name. No one could do a miracle in Jesus’ name unless they were clean from their sins.

We know our record is true because the man keeping it was righteous. He performed many miracles in Jesus’s name—and no one could do such miracles unless they were completely cleansed from sin.