It does me no harm for my neighbor to believe in many Gods or no God. It neither robs my pocket nor breaks my leg.
Who said it: Thomas Jefferson, the third president of the United States and principal author of the Declaration of Independence
Jefferson wrote this in Notes on the State of Virginia
Thoughts on the quote:
Many people, these days, accept religious freedom as the freedom to believe or not. But some do not accept it. This lack of acceptance comes from people of all faiths and no faith. To those of us who believe in freedom of religion, this belief may come naturally. If we are drawn into argument with those who do not agree, we need a reason for our belief in religious freedom. This quote from Thomas Jefferson supplies it.
In addition, it shows the limit of this acceptance: Harm. Physical or economic limits this acceptance. So, if fanatics have beliefs which require them to harm you (examples abound, from the Inquisition to 9/11) then you do have a right to limit those beliefs, but only to the extent that they actually harm you.
This Thomas Jefferson quote reminds me of a story my rabbi, S. Michael Gelber used to tell. It seems a guy was walking down the street, swinging his arms. Hie hits another fellow in the nose. The injured man says "What are you doing? You hit me in the nose!" The arm-swinger replies "It's a free country! I can swing my arms if I want!" To which the injured man replies: "Yes, but your freedom stops where my nose begins."
The Thomas Jefferson quote may then be thought of as a way of judging the lengths of people's noses (metaphorically speaking). Your "nose" includes physical and economic harm. Of course, this is not a full set of criteria, because it can sometimes be hard to judge what economic and physical harm are, and what causes them. But it is a good start.