Why did the forefathers want the freedom of religion in the First Amendment?
England at the time had "The Church of England" because one of the former Kings of England had wanted a divorce and so had created a separate church from Catholicism. If you lived in England, you just about HAD to belong to the Church of England. To do otherwise would assure harrassment by the Kings forces. Much of the original constitution was written in response to the tyrany of the British Crown.
The First Amendment to the U.S. Constitution protects freedom of speech, freedom of religion, freedom of the press, freedom to peacefully assemble, and freedom to petition the goverment for redress of grievances. It prohibits making any law that respects the establishment of religion, but protects the right for people to practice their religion.