Having tasted the power and joy of repentance, they shrink with abhorrence at the thought of engaging in activities that had not only been routine but had had religious significance. Lamoni’s brother proclaims that, should they take up arms again, they would step beyond the power of the Messiah’s atonement. For modern readers, this statement seems extreme, since there is no doctrine that suggests the atonement does not apply to those who defend themselves. Doctrine and Covenants 42:18 does, however, tell us that the atonement cannot cleanse the sinner who is guilty of murder, and Lamoni’s brother has persistently called their former sin as “murder.”
What they did in ignorance and out of mistaken belief can be forgiven; their actions were normal, expected, even commended. But as newly forgiven believers in the gospel, they now have more knowledge. They know that they should not kill in hatred or for religious zeal. In their converted state, what was previously forgivable becomes unforgivable.
Vocabulary: In this context, “retain” means “hold back.” They are not “retaining” their swords in the sense of maintaining possession, but preventing them from being used.