The "Jesus Only" movement, also known as Oneness Pentecostalism or oneness theology, teaches that there is only one God, but denies the tri-unity of God. In other words, oneness theology does not recognize the distinct persons of the Godhead: Father, Son, and Holy Spirit. It has various forms—some see Jesus Christ as the one God, who sometimes manifests Himself as the Father or the Holy Spirit. The core doctrine of Oneness Pentecostal / Jesus Only is that Jesus is the Father, and Jesus is the Spirit. There is one God who reveals Himself in different "modes." Before we look at where Modalism comes from, let's take a look at the biblical view of the Trinity with this illustration:
Trinitarians believe that just like H2O can be water, vapor or ice and be the identical substance, so too with the Father, Son and Holy Spirit. Remember the "Egg illustration" for the trinity? It goes like this: Imagine God as an egg. You have the yolk, the shell and the white part. All the same egg--but very different from one another. The shell is not the yolk, the yolk is not the white, and etc.
If Trinitarians view God as one egg with three parts (white, yolk and shell), Modalists view God as one whole egg with three different names, at different times.
Where did Modalism come from?
Modalism was condemned as heretical as early as the second century A.D, but it is making a big comeback worldwide. What this name means is that God operated in different forms or modes at different times—sometimes as the Father, sometimes as the Son, and sometimes as the Holy Spirit. But passages like Matthew 3:16-17, where two or all three persons of the Godhead are present, contradict the modalism view.
Those associated with this movement are broadly referred to as “Oneness Pentecostals.” Oneness Pentecostalism (also known as Apostolic Pentecostalism or One God Pentecostalism), refers to a grouping of denominations and believers within Pentecostal Christianity, all of whom subscribe to the nontrinitarian theological doctrine of Oneness. This movement first emerged around 1914 as the result of doctrinal disputes within the nascent Pentecostal movement and claims an estimated 24 million adherents today. Not all Pentecostals are in this camp of belief.
A False View of God:
What is a Modalism view of God? Think of a being that can "morph" or "shape shift" from one form into another. There is only one being at any one time. Modalism views God as a being that has three different forms. Sometimes God exists in the "form" the "Father", sometimes the "Son", sometime the "Holy Spirit", BUT NEVER ALL THREE AT THE SAME TIME. The essential difference between Modalism and the Biblical trinity, is that in Modalism, the three members of the Godhood never exist at the same time and in Trinity, they always co-exist at the same time.
Modalists believe that the "role/mode" of the "Son" did not exist before the incarnation and it will cease at the second coming as per 1 Cor. 15:23-28. Modalists teach that generally, God took the role of Father in the OT, the Son during Jesus' life on earth, and the Holy Spirit during the "church age." Which begs the question: "Whom was Jesus praying to?" Modalism, therefore, not only contradicts the Bible, it assaults common sense by destroying the utterly obvious distinction in persons between the Father and the Son.