There it is, the legendary "I'm an atheist" line. Rather, lines. He repeats it. However, I don't see how it's actually relevant to the story. If it's remarkable that Dr. Frank is an atheist, then it should also be remarkable that Papa is Catholic and considers himself the Pope, for example (just spitballing here). For Frank's atheism to be the only mention of any religious affiliation (or lack thereof) is disingenuous.