Actually this isn’t just how one atheist proves Christians don’t think, it’s how many do it. Not all, but many.
It goes like this: “The Old Testament has some really yucky commands in it. If you believe in the Bible you believe in that yucky command. For example, ‘Deformed people can’t come to God,’ or ‘Non-virgins must be stoned with stones,’ etc. But you don’t obey that yucky command, so you really don’t believe in the Bible. In fact, you can’t even stand to think about what the Bible really says.”
Let’s take a closer look at what’s going on here. For convenience, let’s give our atheist the name Richard.
Richard scours the Old Testament for yucky commands. Richard does no contextual study, takes no thought for history, pays no attention to customs of surrounding nations, considers no possible alternate interpretations, does no double-checking of the text’s translation, gives no credence to the progress of revelation (if Richard has even heard of progressive revelation), and most of all consults no Jewish or Christian thinker on the subject.
In other words, Richard presents this OT passage to us having done absolutely no thinking about it whatsoever.
Richard then says to Christians, “This passage means exactly what I think it means.” Implicit (but never stated!) in that statement is, “… without having given it a moment’s thought.”
Richard concludes that because Christians don’t have the same problem with the passage as he does, Christians do not think.
Did you get that? I’ll simplify it, letting “Richard” speak in the first person, but having him voice what’s really going on in his thought processes:
“I conclude without giving it a moment’s real thought that OT passage x means X, which is unthinkable. Since it’s obvious to me that x means X, therefore Christians believe the unthinkable X; therefore Christians don’t think.”
Clear enough yet? I’ll simplify it even further: “My not thinking provides me the evidence I need to prove that Christians don’t think.”