The historical record. The Messiah is to usher in a time of peace, love and reconciliation and the universal love of man for God. Jesus Christ ushered in an era of two thousand years of bloodshed, torture, war and the deliberate infliction of horrific violence on man by man on a scale that the world has never before seen. THAT'S a Messiah? Please.
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Before Jesus there was bloodshed, torture, war and the deliberate infliction of horrific violence on man by man, but it was perceived of as normal.
Jesus said that he would return before the last of his disciples had died. It seems pretty clear that he didn't do this.
An actual Messiah should at a minimum, be truthful. I mean, if you are God, you are supposed to know everything, which includes the future and how to convince all of humanity that one is, in fact, the Messiah.
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Who the Hell knows if there is really a God or not?
God would know that, assuming that there is one.
Also Jesus, the angels, the saints, and Mrs. God (if there is a Mrs God, which the Book of Mormon claims there is.)
I realize that this is a bit of a circular argument.