Originally Posted by DBT
It has not been established that any of the gospels were written by eyewitnesses. Plus there is an overlap, the later gospels copied heavily from Mark, which in turn is based on word of mouth, and possibly earlier unknown manuscripts.

The early church writers from all over the world of that time give testimony on authorship of the gospels. They didn't get together to conspire authorship conspiracies. The critics also quoted these gospels and their authors. A lot of skeptical reasoning today is based purely on speculative conjecture because it is fashionable among the liberals.