tbf many of those sources are in reality just one source. So with the gospels, either Mark is the basis for two of the others or three of them are all based on a fourth source. Then John is really different, almost like he's talking about someone else entirely.
And of course, to the people who use such writings to justify their statements/power/authority, this is a smorgasbord of material to be picked from according to the point you want to make. You can make a large number of entirely contradictory points by selectively quoting from the same religious text. The authority comes from masking the fact that this is what you are doing, asserting that there is only 'one true word'.