When we look at the history of Classical Mythology, we are generally led to assume that the polytheistic religion of the Greeks and Romans faded with the rise of Christianity in the Roman Empire in the 3rd and 4th centuries. Most polytheists willingly converted, though when Emperor Theodosius made Christianity the state religion of the Roman Empire, the remaining pagans living in the empire were violently persecuted. While most historians would say this was the final death of the polytheistic religion of Classical Mythology, I've done a bit of searching online and found that it wasn't the case? That Theodosius failed to exterminate the entire faith, and the pagans continued to practice their religion, despite the attempts of the Church and Eastern and Western Roman emperors to suppress it. Apparently, according to Wikipedia, there were people in Greece still worshiping Zeus as late as the ninth century. Today, there are several revival groups who worship the Greek Gods, in fact they have their own temples and practices. However, it's unclear on when this revival actually started; it just kinda resurfaced when the internet came along.
So, my question is, did worship of the Greco-Roman Gods ever really die out? Or has it survived this whole time and nobody hadn't really noticed. Was there a time when worship of the Gods went completely extinct, and do we know when exactly it was brought back? Or can we never really be sure?
Source: reddit post