I encounter people who reject the Gospels as being historically valuable unless a part of it is supported by another historical source. The assumption is that the Gospels should be seen as false unless there is outside support. I found some quotes from Richard Bauckham (JSNT 31.2 2008) that put this into perspective.
“The burden of proof … usually lies with the doubter. What protects us from gullibility is alertness to reasons for doubting a testimony, but this is not the same as treating all testimony as dubious until we have produced reasons for relying on it. Comprehensive doubt is impossible.”
“[W]e can return to the much more normal historical procedure of assessing our sources as sources. If we consider them trustworthy, then we have to rely on them, without expecting to be able to verify independently everything they say. (There may be special cases where this is possible, but they will be exceptional.) This is a normal and rational procedure, not uncritical gullibility. It is what historians regularly do with sources.”
If you want to learn more about this, I highly recommend Richard Bauckham’s Jesus and the Eyewitnesses.