What are the reasons in Christian theology why it is important that Jesus was fully human? 

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In the early Christian church, a debate arose between those such as Marcion and gnostic groups who argued that if Jesus was divine, he would, like God, not be made of human flesh. Therefore, Jesus only pretended to be a human when he came to the earth in human form, and since he was not human, he did not suffer or die on the cross. This view of Jesus is called docetism. 

That Jesus be fully human is important for several reasons. First, the gospel of John said that he was "the word made flesh." While the canon we call the Bible had not yet been set in the earliest years of the year, John's gospel was widely accepted as authoritative. Therefore, if it--and other widely accepted Biblical accounts-- said Jesus was "flesh," it was important to affirm that. Second, underlying the idea of Jesus as all spirit was the concept that physical matter was evil. Affirming this would negate the Old Testament belief that creation was good, a declaration God makes several times in Genesis. The early church did not want to sever ties with this Old Testament heritage. Finally, if flesh is evil, if Jesus had simply pretended to be human, and if he never died and hence never needed to resurrected, then the resurrection of the body, one of the foundations of Christian theology, would be false. Clearly, this wasn't going to work, so the council of Nicea rejected docetism out of hand and decided that Jesus "was born of the Virgin Mary ... died and was buried and on the third day rose again from the dead."