From what I've heard, the Sincheonji church had a long tradition of sending followers to other churches, hiding their identity, so that they make a lot of friends there, start small clubs and bible studies, and eventually convert them into their own church. (I even heard there were cases when they converted a pastor, winning over a church wholesale, though that might be a bit embellished.)

The problem was so bad that, even before the current epidemic, other "regular" churches had a sign saying "No member of Sincheonji is allowed in our church." So, yeah, not exactly your garden variety Christian church.

When you think about it, this behavior would make Sincheonji a perfect vector for spreading the virus. And that's what happened. Yikes.