In this passage, Joseph Smith emended the singular no one was to the plural none were, probably because of the following plural pronoun themselves. Nonetheless, the original text sometimes uses plural forms in reference to the singular generic one, as in the following instances:
Both of these passages were subsequently edited; for discussion, see under each one. Also note that the first of these (the one involving no one) actually has a mixture of singular and plural usage (first, the plural nouns teachers and ministers, then the singular usage in “except he be a man of God”). The instance of no one followed by themselves here in Alma 6:5 appears to be intended and most probably represents the original text.
Summary: Restore the original singular no one was in Alma 6:5; the generic pronoun one occasionally takes plural referents.