I’m going to assume you are asking about the “Oneness Doctrine” that some teach in opposition to the “Trinitarian Doctrine” held by the vast majority of professing Christians. As you may know the “Trinitarian” is one who believes that “God is one God who at the same time subsists in three Persons: the Father, the Son, and the Holy Spirit.” Those who teach the “Oneness Doctrine” are known as “Oneness Pentecostals” and they teach that “God is one God who manifests Himself in different forms at different times; sometimes as the Father, sometimes as the Son, and sometimes as the Holy Spirit.” In this strange view, God is NOT THREE PERSONS yet He “takes on the form of these three persons one at a time.” They boldly say that “Jesus is God but He is also the Father and the Holy Spirit.”

I have personally met those who espouse this view and I will usually ask them, “So, when Jesus was on the cross and He prayed, ‘Father, forgive them, for they know not what they do’ (Luke 23:34…NKJV), who was He praying to?” They will say unabashedly, “He was praying to Himself.” I will then ask them, “But you teach that God can only take on ONE FORM AT A TIME, so how could Jesus be “the Son” and “the Father” at the same time? Their answers to this question are so ambiguous that I can’t even repeat them. This teaching clearly defies logic, for Jesus was “God manifest in the flesh” (see John 1:1, 14 with 1st Timothy 3:16) and when He prayed that prayer on the cross He was praying to God the Father who was in heaven. He was NOT praying to Himself; He was praying to His Father.

Another prime example of this is Jesus’ prayer in John 17:1-5: “JESUS spoke these words, LIFTED UP HIS EYES TO HEAVEN, and said: ‘FATHER, the hour is come. Glorify YOUR SON, that YOUR SON also many glorify YOU, as YOU have given HIM authority over all flesh, that HE should give eternal life to as many as YOU have given HIM. And this is eternal life, that they may know YOU, the only true God, and JESUS CHRIST, whom YOU have sent. I have glorified YOU on the earth. I have finished the work which YOU have given ME to do. And now, O FATHER, glorify ME together with YOURSELF, with the glory which I HAD WITH YOU before the world was.” If we take these precious words at face value (and we should!), Jesus, the Son of God, was ON EARTH talking to God the Father IN HEAVEN. They are TWO DISTINCT PERSONS, yet they are BOTH GOD. The Son of God had taken on humanity in union with His Deity (see again John 1:1 & 14) and glorified the Father in everything the Father had sent Him to do. In verse 1 He is asking the Father to GLORIFY HIM by raising Him from the dead (after His death on the cross and His burial). In verse 2 He is acknowledging that God the Father has given Him the authority to give eternal life to those whom the Father had chosen. In verse 3 He states that those who have eternal life have the capacity to know the true God (the Father) and the sent one (Jesus Christ, the Son). In verse 4 He declares that He brought glory to the Father from the manger to the cross and completed everything the Father gave Him to do. And finally, in verse 5, Jesus asked to the Father to be with Him again in heaven, “with the glory I had with You before the world was.” In other words, He is asking that His “pre-incarnate glory” (as God!) might be resumed with the Father in heaven! None of His words in this prayer would make any sense whatsoever if Jesus was NOT the Son of God, who had left heaven in obedience to the Father so He could give eternal life to sinners by His death on the cross, and then return to heaven again to be with His Father to enjoy the glory He always had with the Father before He had become a Man. Granted, He will now return to heaven as a TRUE MAN, but at the same time He will be the TRUE GOD (God the Son),  “one of the three Persons who make up the holy Trinity.”

In closing, there are others who deny the “Trinitarian Doctrine” who call themselves “Unitarians.” They teach that “God is only one entity or Person.” Thus they too deny the truth of the Trinity, for in their view, only God the Father is God. Jesus was merely “a man” and the Holy Spirit is a powerful “influence” and not God. To those who hold this error and the false “Oneness Doctrine,” I would simply point them to Scripture where God is revealed as a “Triune (Three-in-One) God” throughout its sacred pages. We have the hint of this truth in the very first chapter of Genesis where we read, “Then God said, ‘LET US make man in OUR IMAGE.” This verse implies that the one true God is made up of at least TWO PERSONS. When we come to the New Testament we learn from the Lord Jesus Himself that God is indeed THREE PERSONS. Jesus told His disciples in Matthew 28:19, “Go therefore and make disciples of all nations, baptizing them in the NAME of the FATHER and of the SON and of the HOLY SPIRIT.” Jesus said “in the NAME” which means they were to be baptized “in the NAME OF GOD.” But then He goes on to teach that “God is a Triune God….the Father and the Son and the Holy Spirit.” Jesus would have been guilty of blasphemy for putting the Father, the Son and the Holy Spirit on EQUAL FOOTING in this verse if each Person was not truly God, co-equal with one another and sharing the same Divine attributes.  (449.3)  (DO)