"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances."
Every election year, religion seems to become a hot topic. We have a bunch of politicians who think that you to be elected, to be a trusted American, you MUST agree with them on religion. Some claim that the US was founded as a "Christian" nation - an example of one of the failures of our educational system.
The first European colonists fell into two broad groups, those fleeing religious persecution and those seeking economic opportunity.
Most countries in Europe had an official state religion. England had forcibly changed religions from Catholic to a newly created Anglican Church of England (Episcopal) at the whims of the King's failed love life. Changing state religions caused massive social and economic chaos. This carried over the British colonies, being Catholic was illegal in Virginia for many years. It was easier to practice Judaism in Virginia, than to be Catholic.
The first Amendment was designed to eliminate this. We don't have, and never should have a connection between religion and government. The government can't endorse, or favor one religion over another. Government money can not be spent on religious purposes. Religious entities can receive government funding for educational or social programs, but they can't require participation in religious activities as a condition of participating the social or educational programs. This is a fine line - a difficult balance. Faith based organizations educate and help millions of people everyday, to do this with government funding, they must make sure that persons of all faiths and persons with no faith, feel equally welcome. If they focus on doing good things, helping people, this can be done.
The free exercise language also assures freedom to not practice a religion. Very early in colonization, people fleeing religion came the Americas, some because they chose not to practice a religion, others who came merely seeking economic opportunity.
We are as much a country built on freedom from religion as we are a country built on religion.