Naturalism proves naturalism therefore naturalism.
This is obviously invalid and nary an atheist can be found who would support such clearly circular reasoning.
However, if you dig a little you can often find that when an atheist has a purported disproof of theism/religion/christianity and he uses “science” as his evidence he often is importing methodological naturalism (MN) somewhere in the mix.
MN should not be treated as evidence since MN isn’t operating as evidence here. It is a method of interpreting evidence, and even a method of perception, assumption, expectations, and thinking. If someone wants to show that naturalism is true one cannot call it an argument to prejudge in one’s methods that there are no supernatural causes. But that’s what MN does if it is used as evidence. It is a prejudicial premise, and therefore circular. It’s invalid. It would be just as invalid for a jury to render a ruling before having the trial.