Short answers, pulled from the story.
Nontrinitarianism is a form of Christianity that rejects the doctrine of the Trinity, which holds that God is three co-equal, co-eternal persons united in one essence. Nontrinitarians include groups such as Jehovah's Witnesses, Oneness Pentecostals, the Church of Jesus Christ of Latter-day Saints, and Unitarians, each holding differing views on the nature of God, Jesus, and the Holy Spirit.
The Trinity doctrine was formally defined at two ecumenical councils: the First Council of Nicaea in 325, which declared the full divinity of the Son, and the First Council of Constantinople in 381, which declared the divinity of the Holy Spirit. The New Catholic Encyclopedia acknowledges that the formulation "one God in three Persons" was not solidly established by council prior to the end of the 4th century.
Michael Servetus was a 16th-century theologian who argued that no scripture explicitly taught the doctrine of the Trinity. The Geneva City Council, in accord with the cantons of Zurich, Bern, Basel, and Schaffhausen, condemned him to be burned at the stake in 1553 for this position and his opposition to infant baptism.
Arianism, associated with the 4th-century presbyter Arius, holds that the Son was the first creation of the Father, subordinate to God and not co-eternal. Modalism, associated with Sabellius around 215 AD, holds instead that God is one person who manifested in three modes: as Father in creation, Son in redemption, and Holy Spirit in regeneration.
The largest nontrinitarian Christian groups include the Oneness Pentecostal movement, the Church of Jesus Christ of Latter-day Saints, Jehovah's Witnesses, La Luz del Mundo, and Iglesia ni Cristo. Despite their size, nontrinitarian denominations together comprise a small minority of modern Christians worldwide.
Isaac Newton, born in 1642, is listed among notable nontrinitarians. In 1733, Voltaire's Letters on the English identified Newton as a member of the antitrinitarians. Several other prominent historical figures have been associated with nontrinitarian belief, including John Locke and American presidents John Adams, Thomas Jefferson, John Quincy Adams, Millard Fillmore, and William Howard Taft.