For an evaluation of the recent comments regarding Trinitarianism, Modalism, and The Elephant Room, see this post.
What Modalism Is
Modalism is an ancient heresy that denies the Trinity by denying that God exists at all times in three distinct persons. While orthodoxy holds to the understanding that God is one in essence and three in person at all times, modalism teaches that God is one but appeared in three different forms of modes at different times. God was Father in the OT, Son during the earthly ministry, and now appears as Holy Spirit.
In ancient times this heresy was known as Sabellianism after its most well-known teacher. It is also sometimes referred to as modalistic monarchianism. While there are subtle differences between these three, for all practical purposes they are one and the same.
Today this heresy is held most notably by the television preacher T. D. Jakes and by the Christian singing group Philips, Craig, and Dean, all of whom are pastors in the United Pentecostal Denomination which is a “oneness Pentecostal” denomination.
Why Modalism is Wrong
Modalism is wrong is because it fails to account adequately for the biblical evidence. The Scriptures are clear that God is one (Deut. 6:4) but also affirms that Father, Son, and Holy Spirit are distinct persons. In John 1:1-2 the Word (the Son) is said to be “with God.” This statement makes no sense if the Son is merely one mode by which the Father manifests himself. Likewise, passages such as 1 John 2:1, which claims that Jesus is our advocate before the Father, and Heb. 7:25, which claims that Jesus makes intercession for his own, make no sense in terms of the plain use of the language. Similarly the Holy Spirit intercedes for believers (Romans 8:27) and is said to have been sent from the Father (John 14:26) which presupposes that the Father and the holy Spirit are distinct persons. Likewise, the Son has said that he must depart for the Holy Spirit to come (John 16:7), which one supposes could imply that the Son must depart in order to return as the Holy Spirit except that the Son specifically says, “If I go, I will send him unto you” (John 16:7).
Why Modalism is Dangerous
Modalism is dangerous because it does not lead people to the God of the Bible. If we accept the ancient creeds as being accurate summaries of the Bible’s teaching about the person of God (and most Christian groups do; in fact, it is one thing that both Catholic and Protestant are in perfect agreement on), then modalism leads people to worship a god different than the God portrayed in Scripture.
Modalism is also dangerous because it allows people to affirm something that sounds like orthodoxy without being orthodoxy. But that’s like poison in your soup, you can’t just spit the bad part out; you have to pour out the whole bowl and start over. Saying, “I believe in the Father, the Son, and the Holy Spirit (as Phillips, Craig, and Dean, do along with T. D. Jakes) is not the same as affirming God the Father, God the Son, and God the Holy Spirit, Three in One, as orthodox Christians affirm.
The Doctrine of the Trinity: No Christianity Without It by Kevin DeYoung (this is an excellent brief overview of the Trinity!)