Why was protecting Religious Freedom important back then?
How do you know?

Assuring that the harms of the British Government wouldn't be repeated

Explanation:

I assume you mean the American Revolution by back then, and the reason that freedom of religion was so important (as well as many other Bill of Rights amendments) was to assure that the harms of the British Government (detailed in the Declaration of Independence) could not be repeated. Many people fled to America to escape the religious oppression of the Church of England, as it was both very strict on what religions were allowed and very closely entangled with the British Government, hence the separation of church and state in America.

spread of christianity