I know it by direct experience and am as sure about it as I am that I am typing on my computer. I do not use the word “know” lightly. I know it the same way that I know that Jesus is the Christ, and I know it the same way Peter knew that Jesus was the Christ: by direct revelation.
Matthew 16
13 ¶ When Jesus came into the coasts of Caesarea Philippi, he asked his disciples, saying, Whom do men say that I the Son of man am?
14 And they said, Some say that thou art John the Baptist: some, Elias; and others, Jeremias, or one of the prophets.
15 He saith unto them, But whom say ye that I am?
16 And Simon Peter answered and said, Thou art the Christ, the Son of the living God.
17 And Jesus answered and said unto him, Blessed art thou, Simon Bar-jona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven.
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I know what I know. You may believe whatever you like.
It’s ironic that you appear to KNOW that mormonism is false, but you BELIEVE that your church is true. Very curious.