Who was the 1st person to say that Jesus was God? I’ve heard a lot of people say that early Christians believed that Jesus was nothing more than a great prophet. Which leads to the question, how do we know that Jesus really is God, and not just a prophet?
That is an easy question. The first person to claim that Jesus was God was Jesus himself. He made this claim, either directly or indirectly, many times. Examples of this include John 8:57-59. When Jesus said "I AM" he was directly taking the name of God. He did not say "I was", but rather "I AM." This is a clear claim of deity. Yaweh, the name God used in Exodus 4, is the Hebrew for I AM. His audience knew exactly what Jesus was claiming here. That is why they picked up stones to stone him.
Jesus claimed to be God again in John 10:25-33. His hearers knew exactly what he was claiming and accused that he had claimed to be God. Jesus did not correct their interpretation, as he surely would have if they were mistaken. Not only did Jesus claim to be God, his twelve apostles made the same claim repeatedly. John 1:1-14, Colossians 1:15-16, Titus 2:13 and dozens of more can be used to show that the apostles claimed deity for Jesus Christ.
Of course, I have heard the spurious claims that the early Christians believed Jesus was just a prophet. There is no basis for this claim. Now, it is true that there were some splinter groups, such as the Ebionites who questioned the deity of Christ, perhaps as early as the late first century. Later groups such as the Arians in the late third century also questioned that Jesus was God, but there is not the slightest evidence that the main thrust of Christianity accepted as the most basic truth that Jesus is God.
As to how we know Jesus was God, I believe his claims and his miracles speak for themselves. Jesus said, "I am the resurrection and the life (John 11:25), after which he raised Lazarus back to life after his body had already begun major decomposition. He said "I am the bread of life," right after he created bread and fish out of thin air. Jesus was raised from the dead. He provided all the evidence one could possibly need to support his claim to be God. Those who choose to ignore this evidence do so for reasons which they will have to explain. Perhaps they believe John, Paul, Peter and all the apostles, and, indeed all 500 eye witnesses to the resurrection were liars. This is an irrational point of view. Jesus was and is God.