The anti-Trinitarian belief that the Father, Son and Holy Spirit are “modes” of God, but not true persons capable of interacting with one another.
The early church initially struggled with the question of whether Jesus was fully God (see Ebionism, Docetism, and Adoptionism) in the same sense as the Father. Once the councils agreed that Jesus is fully God (as is the Holy Spirit), they struggled to reconcile this with monotheism.
Modalism offered a simple and enticing solution. Its proponents taught that God is a mono-personal being that can change “modes,” assuming the role of the Father, Son, and Holy Spirit at different times. Webster’s metaphysical definition for mode is “the form, or way of being, of something, as distinct from its substance.” Modalism teaches that God has one substance, but several “ways of being.” The common illustration that the Trinity is like the forms of water–ice, liquid, and steam–unfortunately is an exact representation of Modalism rather than Trinitarianism.
Although Modalism initially has appeal in its simplicity, it is a dangerous teaching because it misunderstands not only “what” God is, but more importantly, “who” God is and therefore who we are. If the Father, Son, and Spirit are only modes of God (or “masks” as some taught), then the God behind the mask is unknown to us. We are forced to understand the Father, Son and Spirit as illusions and not the true God we desire to know and love. Moreover, if we are God’s children in relation to him as Father, but the Father is an illusion, then our status as his children is also an illusion (Gal 4:6).
Modalism also fails to account for greater than 70 passages in Scripture where the Father, Son and Spirit are mentioned together as distinct from one another and as interacting with one another. In Jesus’ baptism (Matt. 3:16-17), the Father speaks and the Spirit descends upon Jesus. In the Garden of Gethsamane (Mark 14:36) and hanging on the cross (Luke 23:46), Jesus himself prays to Father. John’s Gospel is full of reference to the three person’s relations, not only in their speaking to one another (John 17), but in actions such as “sending” (John 14:24-26; 20:21) and “loving” (John 3:35; 14:32; 15:9). The epistles also show the three persons as distinct. Galatians 1:1 teaches the Father raised the Son; 1 John 2:1 teaches the Christ is an advocate between humanity and the Father; 1 Peter 1:2 teaches different roles for each Person of God in the salvation of man; the relationship between the Father and the Son is given as the modal for the fellowship between believers (1 John 1:3).
Modalism was the most common Trinitarian error in the history of the church and it continues today. Many in the church may inadvertently hold to Modalism without realizing it, but others still openly deny the personal distinctiveness of the Father, Son and Spirit. Groups such as Oneness Pentecostals and the "Jesus only" movement, popular preachers (notably T.D. Jakes), and even some Christian musicians (such as Philips, Craig and Dean, click here for more) are modern Modalists.