There are about 24 million people who belong to Pentecostal groups which embrace Oneness Theology, or Jesus Only, which denies the Trinity. These people believe in what is called Modalism, meaning that there is one God who manifests Himself in different modes, Father, Son, and Holy Spirit. Sometimes He is the Father. Sometimes He is the Son. Sometimes He is the Holy Spirit, but He is never three at the same time.
Orthodox theology teaches that there God subsists as Father, Son, and Holy Spirit. There are three persons in the Godhead, each distinct from the other but one in essence. The attributes of divinity ascribed to the Father, are ascribed to the Son, and to the Holy Spirit. The Athanasian Creed settled the issue for the Church that Jesus is very God of very God, and yet distinct from Him. Athanasius (AD 293-373) was the bishop of Alexandria.
“Whosoever will be saved, before all things it is necessary that he hold the catholic faith. Which faith except everyone do keep whole and undefiled, without doubt he shall perish everlastingly. And the catholic faith is this: That we worship one God in Trinity, and Trinity in Unity, neither confounding the persons, nor dividing the substance.
For there is one Person of the Father, another of the Son, and another of the Holy Spirit. But the godhead of the Father, of the Son, and of the Holy Spirit, is all one, the glory equal, the majesty co-eternal.
Such as the Father is, such is the Son, and such is the Holy Spirit. The Father uncreated, the Son uncreated, and the Holy Spirit uncreated. The Father incomprehensible, the Son incomprehensible, and the Holy Spirit incomprehensible.
The Father eternal, the Son eternal, and the Holy Spirit eternal. And yet they are not three eternals, but one Eternal.
As also there are not three incomprehensibles, nor three uncreated, but one Uncreated, and one Incomprehensible. So likewise the Father is Almighty, the Son Almighty, and the Holy Spirit Almighty. And yet they are not three almighties, but one Almighty.
So the Father is God, the Son is God, and the Holy Spirit is God. And yet they are not three gods, but one God.
So likewise the Father is Lord, the Son Lord, and the Holy Spirit Lord. And yet not three lords, but one Lord.
For as we are compelled by the Christian verity to acknowledge each Person by Himself to be both God and Lord, so we are also forbidden by the catholic religion to say that there are three gods or three lords.
The Father is made of none, neither created, nor begotten. The Son is of the Father alone, not made, nor created, but begotten. The Holy Spirit is of the Father, neither made, nor created, nor begotten, but proceeding.
So there is one Father, not three fathers; one Son, not three sons; one Holy Spirit, not three holy spirits.
And in the Trinity none is before or after another; none is greater or less than another, but all three Persons are co-eternal together and co-equal. So that in all things, as is aforesaid, the Unity in Trinity and the Trinity in Unity is to be worshipped.
He therefore that will be saved must think thus of the Trinity.
Furthermore, it is necessary to everlasting salvation that he also believe rightly the Incarnation of our Lord Jesus Christ. For the right faith is, that we believe and confess, that our Lord Jesus Christ, the Son of God, is God and man; God, of the substance of the Father, begotten before the worlds; and man of the substance of his mother, born in the world; perfect God and perfect man, of a rational soul and human flesh subsisting. Equal to the Father, as touching His godhead; and inferior to the Father, as touching His manhood; who, although He is God and man, yet he is not two, but one Christ; one, not by conversion of the godhead into flesh but by taking of the manhood into God; one altogether; not by confusion of substance, but by unity of person. For as the rational soul and flesh is one man, so God and man is one Christ; who suffered for our salvation, descended into hell, rose again the third day from the dead. He ascended into heaven, He sits at the right hand of the Father, God Almighty, from whence He will come to judge the quick and the dead. At His coming all men will rise again with their bodies and shall give account for their own works. And they that have done good shall go into life everlasting; and they that have done evil into everlasting fire.
This is the catholic faith, which except a man believe faithfully, he cannot be saved.” The baptism of Jesus refutes Oneness Pentecostalism for the Father spoke, the Holy Spirit descended, and Jesus was baptized all at the same time.
“Then cometh Jesus from Galilee to Jordan unto John, to be baptized of him. 14 But John forbad him, saying, I have need to be baptized of thee, and comest thou to me? 15 And Jesus answering said unto him, Suffer it to be so now: for thus it becometh us to fulfil all righteousness. Then he suffered him.
The Holy Spirit
16 And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him:
17 And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.” (Matt. 3:13-17)
Modalism asserts there is one God who can be designated by three names. He can be called Father. He can be called Son. He can be called Holy Spirit. He can be called this at different times. These three are not different persons. They are just different modes of the one God. God can be called Father, as the Creator of the world, and the Law Giver. He can be called the Son, as God incarnate in Jesus Christ. He can be called Holy Spirit, as God in the Church age. Accordingly Jesus can be called God. The Spirit can be called God, but not as distinct persons.
Modalism has been condemned throughout Church history because it attacks the nature of God as revealed in the Bible. In AD 381 at the Council of Constantinople, Modalism was condemned as heresy. If you do not have the true God, you do not have the true Christ, and the true Spirit.