One wonders here if the singular is might be an error for are, especially since the following relative clause reads “which are true”. Perhaps the is was prompted by the preceding occurrence of is in “faith is not to have a perfect knowledge of things”. In his editing for the 1837 edition, Joseph Smith emended the second is to the expected plural are. Nonetheless, the earliest text has numerous instances of singular is for the plural noun things. See, for instance, the discussion and examples listed under 2 Nephi 2:14 for the biblically styled phrase “all things that in them is”. Other examples from the original text include the following sampling of instances of “things which is”:
The critical text will therefore restore the original instance of “things which is” in Alma 32:21: “ye hope for things which is not seen which are true”.
Summary: Restore in Alma 32:21 the original singular is in “things which is not seen”; such usage was quite frequent in the original text of the Book of Mormon.