Well, the belief that Jesus Christ is the son of God is not really a cornerstone of Islam. Jesus, in Islam, was just a messenger of God and the Masih (Messiah) and was sent to guide the Children of Israel with the Injil (the Gospel) and foretold the coming of Muhammad. He was born to a virgin Maryam (Mary) and did perform miracles. However, they think that he was brought up alive into Heaven, like Elijah and Enoch, and not crucified for the sins of the world.
Jesus Christ in Christianity is the son of God, the Messiah, and the only way to Heaven. So, if someone believes that Jesus was the son of God, then they're either a Messianic Jew or a Christian, since that's not a characteristic of Islam.