What you are seeing in the video is not even the full congregation, just some recent graduates.
The Shincheonji Church of Jesus, in the Republic of South Korea, held a graduation ceremony with over 100,000 graduates.
One of the requirements to graduate is to evangelize.
They achieved these numbers through bible study not traditional church as we know it here in the US.
Many of my atheist friends claim that religion is on the downslope.
I’m not convinced.