I have to agree with V and Elsewhere. The backlash would be huge, and any government with that much power would be awful. I am an atheist, but I would join any underground movements to destroy any government that had that much power. People claim that religion causes the evil in the world and that getting rid of it would help. I don't agree. While I feel that religion is foolish, I think that most evil is acted out because of a quest for power. Religion is just one of the many tools people use to control. Government shutting down religion would just be government gaining more power. That is a scary situation. Of course, you'd still have aboriginal tribes, amazonian religions, etc. How do you shut them down?
For instance, I know nothing about the Muslim faith other than what I have seen the extremists preach and do. Based on their actions many people have decided Islam is too dangerous to even exist. Maybe they are right, I don't know. You think that the extremism is bad now? Imagine what it would be like if you outlawed Islam!