I would have to say that the Garleans are probably NOT atheists. Atheists tend not to use words like "blasphemy" and "heresy." Do they not believe in The Twelve? Yes. Do they not believe in the godhood of Primals? Obviously. They do have a religion, though. Just not a religion we have many details on. Best guess would be that similar to Rome, they believe in the sanctity of the Imperial line, and perhaps by extension their government. That is just conjecture at this point, however. Still, they all talk a lot about "heresy" and "blasphemy" with when referring to Eorzeans and Primals and such. They clearly have some sort of organized dogma they adhere to.
