Korihor had a false angel appear to him, he gave him a commission to teach, the end result was putting God out of the reach of man and denying Christ. He didn't know what he was preaching, he didn't know God, but he went on pretending anyway.

"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. And I have taught his words; and I taught them because they were pleasing unto the carnal mind; and I taught them, even until I had much success, insomuch that I verily believed that they were true; and for this cause I withstood the truth, even until I have brought this great curse upon me." (Alma 30:53)

"Nothing is a greater injury to the children of men than to be under the influence of a false spirit when they think they have the Spirit of God. Thousands have felt the influence of its terrible power and baneful effects. Long pilgrimages have been undertaken, penances endured, and pain, misery and ruin have followed in their train; nations have been convulsed, kingdoms overthrown, provinces laid waste, and blood, carnage and desolation are habiliments in which it has been clothed." (Joseph Smith, HC ch. 33, p. 573)