The Church of Jesus Christ of Latter-day Saints has proclaimed to the world consistently since its beginning that there was an apostasy of the church founded by Jesus during his Palestinian ministry and led by his Apostles following his ascension. This is a fundamental belief of the Latter-day Saints. If there had not been an apostasy, there would have been no need for a restoration.
With that mind-set, it's not unbelievable that the Apostate/Apostle was made.