"Modalism... not only contradicts the Bible, it assaults common sense by destroying the utterly obvious distinction in persons between the Father and the son." - Chris Rosebrough
Modalism (Also called Oneness Theology, Modal Monarchism, and Sabellianism) is the Anti-Trinitarian belief that there are not three distinct persons in the Trinity but instead one person who can appear in three different forms (Or modes). They believe that He appears only as three different forms at three different times and is not three unique persons.
Sabellianism is entirely inseparable from Patripassionism, the belief that it was God the Father who died on the cross. This shows that the heresy of Modalism both attacks the nature of God and distorts the Gospel message. It also, along with Arianism, is intertwined with Monarchianism, the belief that there is only one member of the Godhead.
Why is it Heresy?
Modalism is a heresy because it denies the individuality and personhood of each member of the Trinity. In doing so, it distorts the nature of God so much that it sets up an idol and worships a false god.
Sabellianism was first taught in the 200s by Sabellius, Noetus, and Praxeas. Of the three, Sabellius was the last (Showing up decades after the others) and was thought to be the most scholarly and influential. These men were vigorously opposed by Hippolytus and the Montanist Tertullian, and they all eventually ended up being excommunicated. Very few of their writings have survived, and most of what is known from their teachings are actually from polemical pieces written by Trinitarian Apologists. While Tertullian and Hippolytus were the most avid opponents, Dionysus of Alexandria (Dionysus the Great), Dionysus of Rome, Athanasius of Alexandria, and Cyprian all wrote against the heresy. Ultimately this heresy was condemned at the Council of Nicea and again at the First Council of Constantinople.
In the 300s, Didymus the Blind claimed that the Montanist sect adhered to Modalism. He referred to them as "uneducated" and "thick-headed".
In the mid 1500s, Michael Servetus similarly taught the Modalist heresy, along with numerous other heresies. He was one of a minimal number of modalists from the 200s to the 1900s.
In 1913, R.E. McAlistair preached Modalism in a sermon about Baptism at the Apostolic Faith World-Wide Camp-Meeting. After that, Frank J. Ewart taught that all people who are baptized a Trinitarians need to be re-baptized. Some even claim prophecies from that night. This lead to the birth of Oneness Pentecostalism. William Branham, a popular faith healer in the 1900s, was a modalist.
In modern times, Modalism is most prominent in Oneness Pentecostal groups. Adherents to Modalism include Prosperity Teacher T.D. Jakes, Contemporary Music group Phillips Craig and Dean, Oneness Pentecostal Irvin Baxter, and author Tommy Tenney.