I think that non theists should be tax exempt. The reason for this is that theists, most of them, can look forward to some kind of immortality, for some it involves eternal paradise - nice! We, however, are consigned to oblivion or even damned to eternal anguish or worse, torment. Given that they have eternal life and we have just a fleeting moment of mostly hardship, surely the compassionate thing to do would be to say, "Here you are, guys," have this nano moment on us, we've got Eternity of amazingness, you see." So tax exemption would be the first consideration. Fair's fair.

Also, how many people would be really committed to an invisible god if by forsaking him, they'd get 100% tax relief?

If Charity is given to the poor by the rich, then it follows that the time-rich should give to the time-poor - atheists!