The parent explicitly pointed out that religious organizations don't operate outside the law. If they did, literally every corporation and political party in the US would also establish itself as a religion.

What Congress isn't allowed to do is pass a law stating "no religious organizations in the US are allowed to practice human sacrifice." But you still wouldn't be able to appease Xanakto the Moon God because Congress is still allowed to make murder illegal in general, just not exclusively in the context of religious practice.