Short answers, pulled from the story.
Nontrinitarianism is a form of Christianity that rejects the Christian theology of the Trinity. This belief system denies the doctrine that God is three distinct hypostases or persons who are coeternal, coequal, and indivisibly united in one being.
The First Council of Nicaea convened in 325 AD declared the full divinity of the Son. This council marked a turning point where nontrinitarian views faced systematic suppression by Roman Emperors.
After the denominations in the Oneness Pentecostal movement, the largest nontrinitarian Christian denominations are the Church of Jesus Christ of Latter-day Saints and Jehovah's Witnesses. Other smaller groups include Christadelphians, Church of the Blessed Hope, and Christian Scientists.
Arianism held that Arius believed the pre-existent Son was directly created by the Father before all ages. He taught that the Son was brought forth as the very first of God's creations.
Nontrinitarians argue that for a teaching described as fundamental, it lacks direct scriptural support. They point to Deuteronomy 6:4 where the plural form Elohim denotes majesty rather than plurality and cite John 10:30 where Jesus says I and the Father are one meaning unity of purpose.