No one has ever seen God. That is what John 1:18 states, and we see a similar statement again in 1 John 4:12. This can be confusing because we know that God physically appeared to humans on different occasions in the Old Testament, and since Jesus is God, many people saw him during his time on earth. So, how can the Scriptures assert that no one has seen God? The answer is a bit of a riddle, but it makes sense when you think about this with the concept of the Trinity in mind.
The Trinity is made up of the Father, Son, and Holy Spirit. Three in one. Jesus, the Son, is one member of the Trinity. He is not one-third God; he is fully God, as are the other members of the Trinity. It is a hard concept to wrap a human mind around, and likely one we will not have a perfect understanding of until we arrive on the other side of eternity, but it is important to this conversation. When the apostle John wrote that no one has ever seen God, he was referring to God the Father. We know this because v.1 provides a description of Jesus as God, and v.14 tells us he became flesh. This statement makes more sense when we recognize John was referring to the Father.
Who appeared to Abraham in Genesis 18? Who did Moses speak to in Exodus 33? Who did Isaiah see on the throne in Isaiah 6? If no one has ever seen the Father, the only plausible answer is Jesus. Now, some people do not like to say Jesus at this point since he had not yet come to earth as a human. They prefer to use the term pre-incarnate second member of the Trinity. But I just say Jesus because it is easier. In theology, any manifestation of God in the Old Testament is typically referred to as a Theophany, and any pre-incarnate appearance of Jesus is typically referred to as a Christophany. Most theologians agree that the Angel of the Lord in the Old Testament was the pre-incarnate Jesus and that each of these encounters was a Christophany. There is no perfect agreement on this, but the position fits well in the totality of Scripture.
I think it is fair to say that every encounter with God in the Old Testament, including the Angel of the Lord, was Jesus. Some will disagree with that assertion. That is fine, I do not think it is something worthy of debate, but if we look at the totality of Scripture, it fits. And it helps destroy the argument some like to make that Jesus came to earth to save us from the angry, judgmental, vindictive God of the Old Testament.