As used in the Book of Mormon, it means one does not understand and has not accepted true doctrine.1 The word unbelief means to accept false doctrine or to have an incomplete and inaccurate understanding of correct doctrine. Unbelief is often used in conjunction with losing truth, forsaking doctrine, and “dwindling.” The phrase dwindling in unbelief is the Book of Mormon’s way to describe moving from a state of belief, with true and complete doctrine, to a state of unbelief, where the truth has been discarded. Miracles end because men dwindle in unbelief.2
1 “1 Nephi 14:5,” July 6, 2010, blog post comment.
2 Passing the Heavenly Gift, 52.