That may have been a relatively enlightened way to run an empire, but that doesn't mean it was sustainable. The implicit cynicism involved in this cosmopolitan tolerance bred a backlash, namely the more intense, uncompromising spirituality of the Mystery Religions which was combined with Jewish moralistic monotheism to form Christianity, the cult that replaced Roman eclecticism. (Indeed, that backlash seems similar to the current authoritarian one that's undoing cynical consumerist neoliberalism.)

I'm actually writing an article now that goes further into this Roman use of Christianity.