I'm also an atheist, and from my perspective, the U.S. might not be a Christian nation in word, but we are in practice. Christian supremacists seem to think that "freedom of religion" means freedom to choose which denomination of Christianity to belong to - or, if they're feeling generous, Judaism. Any other faith, or lack thereof, is either not taken seriously or seen as outright dangerous.

In contrast, there are European countries with official, government-sponsored Christian churches whose populations are, in practice, largely secular. Reading the book "Society Without God" by Phil Zuckerman opened my eyes to this.