The original text of the Book of Mormon occasionally used the biblically styled second person pronoun thou in plural contexts. Here we have three cases of this nonstandard use of thou. For another nearby example, see Mosiah 2:11. For the three instances here in Mosiah 2:25–26, the 1837 edition replaced the nonstandard thou with ye (and also changed the verb to the appropriate second person plural verb form: are, were, and behold ). The critical text will, of course, restore the original nonstandard usage. For another example, see 1 Nephi 3:29. Also see the complete list under thou in volume 3.
Summary: Restore in Mosiah 2:25–26 the repeated use of thou (and the associated verb forms art, wast, and beholdest) in this plural context; the multiple use of thou in this passage is clearly intended, even though it is nonstandard.