Read more: Which Religions Believe in God?
Judaism and Islam:
The God of the Hebrews is universal and indivisible. This is one of the reasons Jews never create images of God: the infinite cannot be expressed in a mere image. While the Jews do believe a Messiah will one day come, he will be a ordinary person, not a divinity like the Christian Jesus.
Muslims have a similar belief concerning the unity and infinity of God. They do believe in Jesus, and even believe he will return in the end times, but once again he is considered a mere mortal, just like any other prophet, brought back entirely though the will of God, not through any power wielded by Jesus.
Biblical Reasons for Denying the Trinity:
Non-trinitarians deny that the Bible ever states the existence of the trinity and feel certain passages contradict the idea. This includes that fact that Jesus always refers to God in the third person and states there are things that God knows and he does not, such as the date of the end times (Matthew 24:36).
Many arguments in favor of the trinity comes from the Gospel of John, a highly theological and metaphysical book, unlike the other three gospels, which are primarily narrative.
Pagan Precursors of the Trinity:
Non-Trinitarian Groups in History:
Throughout history, multiple non-trinitarian groups have developed. For many centuries, they were condemned as heretics by the Catholic and Orthodox Churches, and in places where they were a minority they were often executed if they did not conform to the wider trinitarian view.
These include Arians, who followed the beliefs of Arius, who refused to accept the trinitarian view at the Council of Nicaea in 325. Millions of Christians remained Arians for centuries until Catholicism/Orthodoxy eventually prevailed.
Various gnostic groups, including the Cathars of the 12th century, were also anti-trinitarian, although they held numerous additional heretical views, including reincarnation.