- Mark was written first
- Matthew and Luke wrote their gospels with Mark as a guide. They also shared another document, "Q", with more stories of Jesus. And each had their own documents, M and L, that the other didn't have access to. Mark, Luke, and Matthew are the synoptic gospels.
- John's gospel is a later, separate creation. He may or may not have had the previous three gospels as guides.
OK, let's look at Jesus's baptism by John the Baptist. It is the first scene in Mark's gospel. Matthew and Luke have the scene also, but they add stories of Jesus's birth and childhood. John's first scene is also the baptism of Jesus by John, although his version tries to make it look like Jesus is not subordinate to John. The gospels in general are a little reluctant to write out this scene clearly because it makes Jesus look like a follower of John. So they hem and haw and write in dialogue to make it seem more palatable.
The fact that it's still in there means it's a basic and early well-known part of the Jesus story, and it's something they can't omit. Even John has to include it, and he has no birth narratives. So a reasonable conclusion to make is that it did happen -- Jesus was baptized by John the Baptist.
So, my reading of this is that Jesus originally was a follower of John the Baptist, one of many who came to him to be baptized. This act was deeply significant to Jesus, and was the beginning of his preaching. Later, especially after John was imprisoned and murdered, Jesus continued his preaching and finally became more popular and significant than John. Some of John's followers switched to Jesus, but probably not all in this early stage. Later the followers of John would join the Way and follow Jesus.