The first amendment says, "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."
It says 'Congress shall make no law respecting an establishment of religion' in the first line, so, since congress can not make laws making us have a certain religion, we have the freedom to choose our religion.
