Where you err is that you keep relating it to distant lands.
Same scenario but instead of China, it's France. You live in Medieval France and suddenly a major city has this huge incident occur where Satanic cultists just blew up the Eiffel Tower and slaughtered thousands of people.
Now, if you're a Satan Cultist, are you going to roam around in Satan Cult robes? And wouldn't the common person be immediately taught what signs to watch put for to ensure that secret Satanic cult doesn't rise again through the ranks of the common class?
Incidences occurred in and around Baldur's Gate involving Shar. She walked in the flesh and fought with Selune in Waterdeep and in the surrounding countryside all throughout the region. She created the Shadow Weave and was directly involved in world changing events.
Imagine Satan came to Earth in the flesh in the land you lived. He started killing and enslaving lots of people. Then God came in the flesh and fought with him in a city not far from where you live, maybe 100 miles away.
The people don't want that again. They would teach everyone what to watch for.
