If you believe anything is permissable without God, this obviously isn’t true if there isn’t a God; since this line of thinking relies on the fact people don’t run around doing whatever they want.
If there is a God, just as obviously, everything is indeed permissible. We are free to do as we will, and history bears this out. If you read scripture, even with God, all things are permissible.
The reason folks don’t run amok has nothing to do with the existence of God. It hinges on a truism of fact and experience that scripture points out: not all things are prudent. How much does it take to figure things out, to be prudential?