From Academic Kids
In Christianity, Sabellianism (also known as modalism) is the second-century belief that the three persons of the Trinity are merely different modes or aspects of God, rather than three distinct persons. It is attributed to Sabellius, who taught a form of this doctrine in Rome in the second century. Hippolytus knew Sabellius personally and mentioned him in the Philosophumena. He knew Sabellius disliked Trinitarian theology, yet he called Modal Monarchism the heresy of Noetos, not that of Sabellius. Hippolytus thought he had very nearly reconciled Sabellius to the mainstream church. However, during the controversy surrounding Paul of Samosata, the Patriarch of Antioch who was deposed in 268 for his Christology, Sabellius' name was mentioned. No one in Antioch had heard of him, but his name was associated with Rome. So they wrote to Rome about him. Pope Dionysius replied in a letter of which only a small fragment has been preserved, but this letter was enough to make Sabellius famous among the Greek theologians.
Another name for this doctrine is Patripassianism from the Latin words patris for "father", and passus for "to suffer". This name was given because the doctrine implies that God the Father came to earth and suffered in the form of God the Son. Some Trinitarians argue that Sabellianism logically leads to Nestorianism.
Historic Sabellianism taught that God the Father was the only person of the Godhead, as do Oneness Pentecostals today. This teaching proports that the identity of God the Father and Jesus is the same. According to this belief, the terms "Father" and "Holy Spirit" both describe the one God who dwelt in Jesus. Some Oneness detractors call this the "Jesus-Only doctrine".