Sabellianism, also known as modalism, is a heresy which states that the Father, Son and Holy Spirit are different modes or aspects of one God, rather than three distinct persons. Modalism's unitarian view of God is commonly described by an analogy: water in its three states of ice, liquid, and steam appear to be different substances, but in reality they all are composed of the same chemical compound, H20. Likewise, for Sabellians or modalists God the Father, Christ, and the Holy Spirit appear to be three distinct persons, but they are really just different manifestations of one solitary God.
The heresy is attributed to Sabellius, who taught a form of this doctrine in Rome in the third century. Hippolytus of Rome, an early Christian writer and martyr, knew Sabellius personally and mentioned him in his Philosophumena (or Refutation of all Heresies). He knew that Sabellius disliked Trinitarian theology, yet he called Modal Monarchism the heresy of Noetos, not that of Sabellius. Sabellianism was embraced by Christians in Cyrenaica, to whom Demetrius, Patriarch of Alexandria, wrote letters arguing against this belief.
The chief opponent of Sabellianism was Tertullian, who labelled the movement "Patripassianism," from the Latin words patris for "father", and passus for "to suffer" because it implied that the Father suffered on the Cross. The term was coined by Tertullian in his work Adversus Praxeam, Chapter II, "By this Praxeas did a twofold service for the devil at Rome: he drove away prophecy, and he brought in heresy; he put to flight the Paraclete, and he crucified the Father." (However, it should be noted that the term in this context is used as an epithet against Praxeas, an early anti-Montanist, and not here applied to Sabellianism.)
Sabellius was excommunicated as a heretic by Pope Calixtus I in AD 220.
Today, Sabellianism, despite being rejected by the Church for nearly 1800 years, is accepted primarily by some Pentecostal groups, who are sometimes referred to as Oneness Pentecostals or "Jesus Only" Pentecostals.