So, leaving out the born-ins, because they are brainwashed from birth through no fault of their own and are mostly innocent in all this, the jovies get new converts through a Nigerian-type scam. However, that said, the converts are either gullible or, in some strange way, wanting to be conned. They actually want to believe in all the jovie horseshit because, for them, the thought of being dead for the rest of eternity is just too frightening to contemplate.
It also strikes me that what we have here is some kind of perverse symbiotic relationship between the jovie converts and the jovie faith. The jovie faith needs the converts just as much as the converts need the faith,
This also explains the great sense of loss that a jovie feels when they leave the jovie sewer.