The rule could be anything, as funny or as serious as you want. The universe will progress in a similar way that it has up until this point, unless your changed rule prevented it from doing so.
Some examples might be:
- The invention of currency is not allowed.
- Iron is slightly less stable.
- The Ancient Greeks are able to cultivate Silphium, which does not go extinct now.
I’d create an actual god with morals of absolute good that rules over humans with an iron fist. Political debates? There’s an objective answer and the god knows it. Do something wrong? Get warned. Do something very wrong? It’s brainwash time.