Why can't I voice my thoughts against religion?

Every day, you hear of people basically advertising Religion. Mainly in the Western world, Christianity. As a christian, you seem to have a right to voice your beliefs on God anywhere you like, for instance, in a laboratory. So why can't I voice my thoughts on the nonexistence of God in a church or something? They can in my places of belief & meaning (labs & Universities). So why can't I?