John clearly outlines that Jesus is God in Chapter 1 of his gospel: "In the beginning was the Word, and the Word was with God, and the Word was God...And the word was made flesh and dwelt among us and we saw His glory, the glory of the only-begotten of the Father, full of grace and truth."
By that definition, Islamists and Jews who all agree Christ existed are Christians - which clearly they are not. Jesus is the Son of God, the Word of God made flesh, if you read the bible, you will find in the 1st chapter of John's Gospel, that He is God.