Who did Moses see when he saw the backside of God? He was seeing the pre-incarnate Christ. Let's take a look.
"Then I will take My hand away and you shall see My back, but My face shall not be seen." Exodus 33:23
Jesus said in John 6:46, "Not that any man has seen the Father, except the One who is from God; He has seen the Father." Jesus had stated that no one had seen the Father. This is also what Paul says in 1 Tim. 6:16, "who alone possesses immortality and dwells in unapproachable light; whom no man has seen or can see. To Him be honor and eternal dominion! Amen." The Holy Spirit appears as a flame or wind but never in any human form. Therefore, when we realize that God was seen in the Old Testament, it was not the Father or the Holy Spirit. It would have been the pre-incarnate Christ. Consider these verses:
- Ex. 6:2-3, "God spoke further to Moses and said to him, ‘I am the LORD; and I appeared to Abraham, Isaac, and Jacob, as God Almighty, but by My name LORD I did not make myself known to them.'"
- Exodus 24:9-11, "Then Moses went up with Aaron, Nadab and Abihu, and seventy of the elders of Israel, and they saw the God of Israel; and under His feet there appeared to be a pavement of sapphire, as clear as the sky itself. Yet He did not stretch out His hand against the nobles of the sons of Israel; and they beheld God, and they ate and drank."
The Bible tells us that God was seen, yet we have in John 1:18, "No man has seen God at any time; the only begotten God, who is in the bosom of the Father, He has explained Him." In the gospel of John, four verses earlier in verse 14, it said that the word (which was God, John 1:1) became flesh and dwelt among us. From then on, whenever John the apostle refers to "God" he is referring to the father--because the Word had become incarnate in the man Jesus.
Therefore, we can see that whenever God is seen in the Old Testament, it is the pre-incarnate Christ. Moses was seeing the Word before it was made flesh or became flesh in the Old Testament.