53 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.
Be careful what you say, because you can convince yourself of the things you teach. Why is it so important for us to testify? Because we reinforce our belief in core principles of the gospel by verbalizing them. Why is sarcasm dangerous? Because we are putting into words ideas that we do not believe in, which creates inner conflict. Why should we not teach false doctrines? Because, besides the damage we do to others, we may eventually come to believe our own message.
There is another element of this story that is troubling: why did Korihor never mention the angel previously? If he really was sincere, why was he so afraid to mention the angel in his teachings? Clearly, he thought that attributing his knowledge to any kind of supernatural experience would weaken his argument that God does not exist. If you have to hide part of your story to make it work logically, that is a red flag to you that you are not on a solid foundation.