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Pray today your Mormon friend or relative would look in their Bible to see that Moses actually died and was buried. Share with them from the end of Deuteronomy how this happened, and that he was indeed dead. By using demonstrative evidence, you can back up your words through the Bible. Also ask where in the Bible does it say Moses took part in the first resurrection with Christ?
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“It is clear that he was translated and taken into heaven without tasting death so that, with Elijah, another translated being, he could return on the Mount of Transfiguration and give keys and authority to Peter, James and John. (Matt. 17:1-13; Alma 45:18-19; Teachings, p. 158…) And we know that he had part with Christ in the first resurrection.” Mormon Doctrine, p. 515.