I don't know if you know this or not, but the empire was going to fall regardless of religion. We don't know what would have happened after the fall though if there wasn't religion. And by religion, I assume you just mean Christianity, since Islam during the European dark age was still growing and advancing in technology. So basically, you believe that if Christianity didn't exist, humanity would have excelled and recovered from the fall and advance in technology much sooner than it already did. You'd still be wrong, since religion was still a thing before Christianity. Romans (before Christianity) believed in polytheism, and they still advanced and grew.