It may be true that we have areas in our brain that evolved not only 'consistent with holding religion', but actually evolved 'specifically for the purpose of experiencing religion'... but it would be very hard to prove this second statement, and anyone who makes it should be highly suspect.

To prove the second statement, we just need to find gene variants that are strongly correlated with religious beliefs.

No, the second statement hinges on a causal claim, so correlations alone can't prove it unless supplemented with strong causal assumptions. Gene variants being correlated with religious beliefs is consistent with three different causal hypotheses: (1) gene variants influence religious belief, (2) religious belief influences gene variants, and/or (3) gene variants and religious belief have some common cause. Correlations only tell us that at least one of the hypotheses is true; they don't allow us to conclude that hypothesis 1 is correct.

ETA: [does Fonzie thumbs-up] aaaayyyy!