Source: Trinity Robert Morey, Chapter 22, p. 507ff
The heresy of Modalism is not as well known as Arianism. It has been called by many different names down through the
centuries: Sabellianism, Patripassianism, Monarchianism, Modalistic Monarchianism, functionalism, Jesus Only, Father Only,
and Oneness Pentecostalism. It was one the earliest heresies to afflict the Christian Church. Since most people have never
heard of Modalism, the first thing we must do is explain what it teaches and how it relates to the doctrine of the
Trinity. But this is not as easy as it seems. It is one of the most confusing and self-contradictory heresies in the
history of the Christian Church.
From the beginning, Modalism was based on the Platonic doctrine that God was an indivisible Monad and could not be divided into three separate Persons. Thus, the Father, the Son, and the Holy Spirit are not to be viewed as three distinct Persons, but as three different manifestations, modes, administrations, disguises, roles, or offices of one and the same Person.
When an actor puts on different masks or disguises in order to pretend to be several different characters during a play, he does not become more than one person. He is still one and the same person behind those masks. In the same way, according to Modalism, the "Father," "Son," and "Holy Spit" are only three different masks or disguises which God puts on in the theater of history. There is only one Person behind the masks of the Father, Son, and Holy Spirit.
If this is true, then how can the Bible describe the Father speaking to the Son in such places as Matthew 3:17? If there is only one Person in the conversation, is He talking to Himself? The Modalist's typical answer is that the divine nature or the spirit of Jesus is talking to the human nature