Why do we forget this part of the 1st Amendment?

Next time you have a discussion about seperation of church and state with a progressive (e.g. nativity scene at school, town hall, etc.), ask them to quote the part of the 1st Amendment relating to religion. They should give you something similar to "Congress shall make no law respecting an establishment of religion." Then you can tell them that the clause does NOT end with a period but with a comma and includes "or prohibiting the free exercise thereof;" Ask then if you may freely exercise your religion in a public place with whatever activity they are trying to suppress.