Because, if you study church history you will learn that the dedication of the first temple at Kirtland, Ohio featured, appearances by Jesus Christ, Moses, Elijah, Elias, and numerous angels; speaking and singing in tongues, often with translations; prophesying; foot washing; and other "spiritual experiences."
Evil Gentile critics point out that the Saints had ginned up (pun intended) a mess of home brew and got loaded to the gills. The Holy Saints would later deny their own records and eyewitness accounts. But in any case, there is only so much of this kind of holy rolling that a body can take.
And now you know.
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Perhaps you don't believe the Apostles of the 1st Century that said they saw many of the dead saints walking around in Jerusalem on the Easter morning. Perhaps you don't believe the Apostles when they say they saw the dead Christ appear to them, teach them and eat with them. Maybe you don't believe the 500 or so people that saw the “dead” Christ speak to them and then ascend in to Heaven.
Just because you or I don't believe something doesn't make it not true.
The Apostle Paul, who also heard and saw the “dead” Christ and said that by their fruits you shall know the followers of Christ, Mormon fruit looks pretty good. We could use a lot more of their fruits in this country right now.