Oneness Pentecostal theology teaches that God exists as one person who revealed Himself in three modes or forms. In the Old Testament, God revealed himself as the Father. When Jesus was on earth, the revelation was as the Son. Now, God is in the mode of the Holy Spirit. Basically, oneness teaches three consecutive modes of God: the Father who became the Son who became the Holy Spirit. There are variations on this model within Oneness churches, but I will focus on this model here.
However, in the Old Testament, there are numerous places where God is seen. In some places this is the angel of the Lord. In others, it is a vision or a dream. But, there are instances in the Old Testament where God is seen, and it is not an angel, a vision, or a dream. Of course, this can raise some warning flags for Trinitarians as well as Oneness people. But when you look at the totality of Scripture, you'll find that the Trinitarians have an easy answer where the Oneness Pentecostal people do not.
Basically, in those places where God was seen in the Old Testament, it was the person of Jesus; that is, it was the pre-incarnate Word that was seen. It was not the person of the Father that appeared in the OT because Jesus said that no one has ever seen the Father (John 6:46). Yet, God Almighty was seen (Exo. 6:2-3). For the Oneness people, this is a problem since God was in the mode of the Father in the Old Testament; and to them, it had to be the Father.
- John 6:46, "Not that any man hath seen the Father, save he which is of God, he hath seen the Father."
- Exo. 6:2-3, "And God spake unto Moses, and said unto him, I am the LORD: 3And I appeared unto Abraham, unto Isaac, and unto Jacob, by the name of God Almighty, but by my name JEHOVAH was I not known to them."
- Num. 12:6-8, "And he said, Hear now my words: If there be a prophet among you, I the LORD will make myself known unto him in a vision, and will speak unto him in a dream. 7My servant Moses is not so, who is faithful in all mine house. 8With him will I speak mouth to mouth, even apparently, and not in dark speeches; and the similitude of the LORD shall he behold: wherefore then were ye not afraid to speak against my servant Moses?"
- See also, Exo. 24:9-11; Gen. 17:1; 18:1; 19:24 with Amos 4:10-11; Acts 7:2.
As you can see from the above scriptures, God Almighty was seen; but it was not God the Father. How then can Oneness theology be correct if God was in the mode of the Father in the Old Testament, that God was seen, and yet Jesus said the Father was not seen? The only answer the Oneness people can give is that God appeared as an angel or in a vision or dream. But if that is so, then is an angel God Almighty? Look at Exo. 6:2-3. God identifies Himself as Jehovah (His self-given name) and states He appeared to Abraham, Isaac, and Jacob as God Almighty. Was that an angel? Was it an angel who said, "I am God Almighty"? No. It was God Almighty. Or look at Num. 12:6-8 where God Himself declares that He does not appear to Moses in a vision or a dream, but that Moses beholds His very form. This negates the possibility, at least in this scriptural occurrence, that God appeared to Moses in a vision or dream.
God was seen in the Old Testament, but it was not the Father. It was the Son, the pre-incarnate Christ. Therefore, the Son existed at the same time as the Father in the Old Testament, and Oneness theology is shown to be incorrect because we have both the Father and Son existing at the same time.