A "flash mob" is a large group of people who assemble suddenly in a public place, perform an act for a brief time, then disperse. I wanted to host a flash mob ministry in which we gather and sing and dance to a Christian song then disperse. Is this legal and if not what about it is not legal or what can be done to make it legal?
It's probably legal.
Remember that there is always a legal "tension" between (a) free speech rights and (b) time and manner restrictions. Don't hold your rally at 3:00 am. Don't block traffic. Don't occupy a part of a public park where others have gotten a permit for that afternoon. Don't yell "fire" in a crowded theater. Don't recommend the overthrow of the government.