Modalism, also called Sabellianism, is the unorthodox belief that God is one person who has revealed himself in three forms or modes in contrast to the Trinitarian doctrine where God is one being eternally existing in three persons. According to Modalism, during the incarnation, Jesus was simply God acting in one mode or role, and the Holy Spirit at Pentecost was God acting in a different mode. Thus, God does not exist as the Father, Son, and Holy Spirit at the same time. Rather, He is one person and has merely manifested himself in these three modes at various times. Modalism thus denies the basic distinctiveness and coexistence of the three persons of the Trinity.
Modalism was condemned by Tertullian (c. 213, Tertullian Against Praxeas 1, in Ante Nicene Fathers, vol. 3). Also known as Sabellianism, it was condemned as heresy by Dionysius, bishop of Rome (c. 262).
Modalism is probably the most common theological error concerning the nature of God (i.e., who God is). "Present day groups that hold to forms of this error are the United Pentecostal and United Apostolic Churches. They deny the Trinity, teach that the name of God is Jesus... modalist churches often accuse Trinitarians of teaching three gods. This is not what the Trinity is. The correct teaching of the Trinity is one God in three eternal coexistent persons: The Father, the Son, and the Holy Spirit."