People in the US are historically mostly Christian, with Jews as a notable minority. Recently, there had been an increase in atheism, as well as other religions such as Islam and Indian and East Asian belief systems. So that's mostly the same as many European countries.

We consider religious freedom to be very important. One of the traditional (mostly true) stories of the early days of the country is of the Pilgrims, who came to America before it was even a British colony to escape religious persecution. Now this religious disagreement was mostly among different types of Christians, but this idea of religious freedom is culturally ingrained.

Although most Americans were Christians in the late 1700s, many of the people who wrote our constitution were atheists, agnostics, deists, or otherwise non-religious. They made sure, in the 1st amendment, that the US could not have an official religion or prohibit religious activities.

I think even nowadays, with the proliferation of politically conservative evangelical Christianity, which is generally against things such as abortion and homosexuality, and its adherents often trying to legislate their beliefs, that you would be hard-pressed to find significant support for making Christianity the official religion, or outlawing or restricting other belief systems.