The view that the three members of the Trinity are different modes of God's activity rather than separate Persons.
TH The view that the Son and the Holy Spirit are 'modes' of the Father, or aspects of the One God. Modalism was the first major heresy with which the Early Church contended. Modalism appeared in various guises in the first two centuries of the Church. The Early Church Fathers argued firmly against this view of Christ, which denied a separation of Person between Father, Son, and Spirit. In recent years, Oneness Pentacostal Churches and the "Jesus Only" movement have promoted a Modalistic Christology, arguing that Jesus is actually the Father and the Spirit, not personally distinct from them.
An incorrect view of God, where God changes modes over time, from Father to Son (when Jesus Christ was here) to Holy Spirit (after Jesus' ascension). Thus, modalism denies the trinitarian nature of God. Believed by Oneness Pentecostals, William Branham, Witness Lee, and others.