for ease of typing, the following was pulled from: https://en.wikipedia.org/wiki/Korihor (also see Alma chapter 30)

Korihor taught “that there could be no atonement made for the sins of men, but every man fared in this life according to the management of the creature; therefore every man prospered according to his genius, and that every man conquered according to his strength; and whatsoever a man did was no crime”

“Korihor is directly referred to in The Book of Mormon as an Anti-Christ, because he claimed there will be no Christ. Despite a freedom of religion, Korihor’s views and teachings alarmed the clerical government, who felt that his views were dangerous to their society. ”

“This culminated in Korihor being miraculously rendered deaf and mute, upon which Korihor confessed, in writing, …I always knew that there was a God. But behold, the devil hath deceived me; for he appeared unto me in the form of an angel, and said unto me: Go and reclaim this people, for they have all gone astray after an unknown God. And he said unto me: There is no God; yea, and he taught me that which I should say.”

That leads me to believe the Humanist Manifesto originated from the same source.