My earliest memories of school included learning about how the United States was founded. Different groups of people wanted to be able to worship how they pleased without the government getting into their business.
There was a lot of emphasis put on freedom of religion.
Nobody said to us "you should practice a religion" but it was implied. Why would thousands of people risk their lives and their futures for religion if religion wasn't worth it? I understand now that there were many other factors and freedom of religion wasn't in the original Constitution, but added later. Still, religion is embedded in our story.
What you don't hear about in school is how varied the beliefs of the founding fathers were. And fortunately, for all of us, they crafted the constitution in such a way that would keep any one of the religions of the time from being a part of the government.
It frustrates me to hear my own family members either say we are a "Christian Nation" or, even worse, actually wish we were a theocracy! My neighbor's car is covered with anti-religion bumper stickers including one I'd like to send to my relatives: If You Want a Country Rules By Religion Move To Iran!
Given that religious parents pass their religion down to their children, and we learn religion must be important because it was one of the reasons for founding this country, it makes sense that so many Americans are religious. As a co-worker pointed out to me Europe is mostly secular because they've gone through their enlightenment after almost 2000 years of rule by religion. And since many of the religious...um...kooks...left Europe and founded this country I understand why we are the way we are.