Here the antecedent for the relative pronoun which is a plural noun phrase (“the plain and most precious parts”), but the intervening prepositional phrase has a singular noun phrase (“the gospel of the Lamb”). This nearer noun phrase may be the reason why the singular form hath appears in the relative clause (“the gospel of the Lamb which hath been kept back by that abominable church”). An equally possible explanation is that the original text of the Book of Mormon allows the inflection ending -(e)th for verbs in the third person plural.
In his editing for the 1837 edition, Joseph Smith edited the hath to the standard plural have. The critical text will, of course, restore the original hath. For a similar example, see 1 Nephi 13:34. For a general discussion, see inflectional endings and subject-verb agreement in volume 3.
Summary: Restore the original hath in 1 Nephi 13:32; the biblical -(e)th ending commonly occurs as a plural verb form in the original text.