In conservative America—the real America, as some would have it—nonbelief is a liability. Everyone is supposed to be a Protestant, though this might include Catholic Protestants or Muslim Protestants. You are free to go to the church of your choice, but it is very important that you do go to a church of your choice. Protestant individualism still requires public order and personal moral discipline. And this is best provided by a strong religious foundation.
In liberal America, nonbelief is more acceptable. But liberals allow this because they assimilate nonbelief, as well as just about any religion, into a Protestant conception of faith. That is, even the most uncompromising atheism might be well and good, as long as it is held as a form of personal faith. Matters of the gods, liberals think, are decisions of personal worldview and metaphysics. As such, they are private matters, to be sharply distinguished from public matters. When nonbelievers argue that the gods are fictions, basing such arguments on publicly available reasons and evidence, they step out of bounds.
I run into this liberal Protestant framing especially when observing debates over science and religion. Consider the controversy over evolution. The mainstream view is that evolution is good science, and hence public. Good citizens should come to accept such public, verified facts as presented by science. But to suggest that evolution is anything but fully compatible with proper religion (as understood by liberal Protestants) is unacceptable. That would violate the boundary between what is public and private, faith-based worldviews. Those atheists who argue that evolution counts against the gods are out of bounds, while those nonbelievers who think the reality of gods should be decided entirely on metaphysical grounds are models of rationality. If you suggest science has a vital contribution to the debate over supernatural realities, you’ll get accused of “scientism” or “reductionism” or any number of terms that seem vaguely abusive but otherwise hard to pin down. But if you propose to leave matters entirely in the court of armchair philosophizing, with the implication that nonbelief is a faith-based position on all fours with any other, you’ll be seen as the soul of reasonability.
Catholics, Muslims, and others who belong to religions with a strong communal sensibility, often chafe at the requirement that they fit into a Protestant conception of being religious. Nonbelievers might also consider a bit of rebellion. Fitting a Protestant pattern of personal faith allows us a good deal of social breathing space. But skepticism toward the very idea of faith is an important part of most nonbelievers’ intellectual orientation. Being granted grudging acceptance as long as we treat nonbelief as a form of personal faith becomes grating after a while.