Modalism is a form of the heresy of Monarchianism that appears in a number of variations in the second and third centuries. Modalism is also known as Sabellianism and Patripassianism.
Modalism expresses a denial of the Trinity by stating that God is a single person who has revealed himself in three forms, or modes. The belief is that during Old Testament times in biblical history God manifested himself in the mode of the Father. Then, at his incarnation as Jesus he revealed himself as the Son, and then, after Jesus’ ascension, he revealed himself in the mode of the Holy Spirit at Pentecost. The modes appeared successively and never simultaneously and never existed as the Father, Son, and Holy Spirit at the same time. In this, Modalism denies the fundamental distinctiveness and coexistence of the three persons of the Trinity.
Modalism itself had a number of variants as the heretics attempted to explain aspects of the Trinity under the Monarchic claim. One variant was taught in Rome by a priest Praxeas from Asia Minor around the year 200. He propounded Christ as simply a man who was both the Father and the Son such that the Father suffered with the human Jesus. The belief was condemned by Tertullian in his Against Praxeas written about 213. In this form the heresy became known as Patripassianism (from the Latin pater; father and passus; to suffer) as it teaches that the Father became incarnate and suffered and died on the cross.
Another variant was taught in Rome by Sabellius in which he seemed to have taught the existence of a divine monad that projected itself successively as Father, the Creator; Son, the Redeemer; and Holy Spirit, as the giver of grace, all modes of the same divine person. This form of modalism has been given the name of Sabellianism. The bishop of Rome, Dionysius, condemned this heresy about 262.