Genesis
Introduction

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Genesis 4:15

And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.

God is worried. He says whoever kills Cain will be punished sevenfold (whatever that means). Just to make sure, though, God puts a mark on Cain so no one will kill him. Good idea. But it contradicts the law given in Gen.9:6 that says whoever kills shall be killed.

Until the 1978 revelation to Mormon President, Spencer W. Kimball, black men were not allowed to hold the priesthood. This was based, in part, on Genesis 4:15, where God placed a mark on Cain after he had killed his brother Abel. Joseph Smith and later LDS leaders interpreted the "mark upon Cain" to be turning of Cain's skin (and all of his descendents) black.