In Surah 5- 116 Jesus answer to Allah is a denial that He has said to others that He, Himself, and his Mother were gods equal to Allah. It is this passage that founds their belief that only Muslims are Monotheists; and Christians are polytheists,believing Jesus is also God. They do not recognize the Christian belief; that Jesus is one of three personalities of God, thereby denying the Trinity.
They also muddle the Trinity by making it seem like Father, Mother and Son.