Many Christians including Roman Catholics, Anglicans and many Protestants, believe in the concept of Trinitarianism. They believe that God the Father, is the same as God the Son, Jesus, and the Holy Spirit. They argue that they are separate Beings, yet insist that they are One.
Yet other religions, including Islam and even members of other Christian churches see this as a form of Polytheism, not Monotheism. These non-trinitarianists include Christadelphians, Jehovah's Witnesses, The Church of Jesus Christ of Latter-day Saints, and Unitarians.