Let me make sure I understand...are you arguing that a law mandating the worship of God would be constitutional?
We have a Constitution of negative rights. Therefore the only thing the Constitution can do is bar an attack on religion - not mandate one.
Atheism is not a religion - it is the rejection of religion. Therefore it does not come under th auspices of Constitutional protections, because it is something that is not done, rather than something that is done.
To flip the concept wold destroy the Consititution. It would be to legalize the government power to punish someone for what they don't do. Like if you were punished for not buying healthcare insurance, for example.
Oh wait...
Oh, no...