I was asked to comment on an old youtube personality of TrueEmpircism’s open affirmation of Modalism. He seems out of the Theological loop and I suppose he invests too much money into books on biology and needs to spend more on theology. I won’t try an article rebutting Modalism, but I will quote Theologians on the issue:
At various times people have taught that God is not really three distinct persons, but only one person who appears to people in different “modes” at different times. For example, in the Old Testament God appeared as “Father.” Throughout the Gospels, this same divine person appeared as “the Son” as seen in the human life and ministry of Jesus. After Pentecost, this same person then revealed himself as the “Spirit” active in the church. This teaching is also referred to by two other names. Sometimes it is called Sabellianism, after a teacher named Sabellius who lived in Rome in the early third century A.D. Another term for modalism is “modalistic monarchianism,” because this teaching not only says that God revealed himself in different “modes” but it also says that there is only one supreme ruler (“monarch”) in the universe and that is God himself, who consists of only one person.
Wayne Grudem. Systematic Theology (Kindle Locations 8251-8257). Zondervan. Kindle Edition.
The fatal shortcoming of modalism is the fact that it must deny the personal relationships within the Trinity that appear in so many places in Scripture (or it must affirm that these were simply an illusion and not real). Thus, it must deny three separate persons at the baptism of Jesus, where the Father speaks from heaven and the Spirit descends on Jesus like a dove. And it must say that all those instances where Jesus is praying to the Father are an illusion or a charade. The idea of the Son or the Holy Spirit interceding for us before God the Father is lost. Finally, modalism ultimately loses the heart of the doctrine of the atonement—that is, the idea that God sent his Son as a substitutionary sacrifice, and that the Son bore the wrath of God in our place, and that the Father, representing the interests of the Trinity, saw the suffering of Christ and was satisfied (Isa. 53:11). Moreover, modalism denies the independence of God, for if God is only one person, then he has no ability to love and to communicate without other persons in his creation. Therefore it was necessary for God to create the world, and God would no longer be independent of creation (see chapter 12, above, on God’s independence).
Wayne Grudem. Systematic Theology (Kindle Locations 8264-8271). Zondervan. Kindle Edition.
In Scripture, however, it is clear, as we have seen, that Father, Son, and Spirit are all divine and, further, that they are distinct from one another. That should be obvious from all the many personal transactions among the persons. The Father appoints the Son to a place of honor (Pss. 2:7; 110:1). Father and Son know each other (Matt. 11:27), but the Son is somehow ignorant of something the Father knows (Mark 13:32). The Word is with God, as well as being God (John 1:1–2). The Father gave his Son to die for sinners (John 3:16; Gal. 4:4–6). Jesus prays to the Father (Mark 14:36; John 17; many other places), making requests, giving thanks, expressing love.666 He teaches the disciples to pray to the Father in the name of Jesus (John 16:23). Jesus asks the Father to send the Spirit, who is “another” Paraclete distinct from Jesus himself (John 14:16). The Father speaks from heaven, testifying to the Son (Matt. 17:5). Jesus ascends to the Father (John 20:17) and sits down with the Father on the Father’s throne (Rev. 3:21). The angelic chorus ascribes salvation to God and to the Lamb (Rev. 7:10). None of this makes sense on a modalistic basis. The members of the Godhead are distinct persons.
Frame, John M.. Systematic Theology: An Introduction to Christian Belief (Kindle Locations 13234-13243). P&R Publishing. Kindle Edition.