Its story
Modalism, also called 'modalistic monarchianism' or 'Sabellianism', arose in the late 2nd century out of a desire to protect God's oneness without lowering Christ's divinity. Its solution was to identify the Father and the Son: the one God appears as Father in creation and the Law, as Son in the incarnation and as Spirit in sanctification. Critics called it 'patripassianism', because it implied that the Father himself suffered on the cross.
Noetus of Smyrna taught this around 200 and was rejected by his city's presbyters; his ideas reached Rome, where Epigonus, Cleomenes and Sabellius spread them. Praxeas, known only from Tertullian's reply, brought it to Rome and North Africa; Tertullian wrote that Praxeas 'drove out prophecy and brought in heresy; he put to flight the Paraclete and crucified the Father'. Pope Callistus excommunicated Sabellius around 220, although Hippolytus accused Callistus himself of leaning toward modalism.
The answers of these writers helped shape Trinitarian vocabulary: Tertullian introduced Latin terms such as trinitas and persona (one substance, three persons). In the East, fear of modalism explains why many 4th-century bishops distrusted homoousios, which could be read as sheer identity of Father and Son. The Cappadocian Fathers clarified the distinction between ousia (what is common) and hypostasis (what is proper to each person), and the Council of Constantinople (381) sealed that faith. Marcellus of Ancyra, accused of a new modalism, prompted the creedal phrase 'his kingdom will have no end'.
What is known of Noetus, Praxeas and Sabellius comes almost entirely from opponents, who do not always distinguish between them. Today the 'Oneness' theology of some Pentecostal churches is often compared to modalism; its proponents usually nuance or reject that identification.