There's a very large group of sect members that BELIEVE in this faith. Acting now will not harm us, it can only improve our current life style. If we don't start pushing this faith, other problems will eventually arise such as catastrophic event.
Believe in faith, it can only bring good in the end. If it's a big scam, at least we know it's not celestial villain paying for it.
I would argue that the truth very much matters. To run with your analogy, the doctor is not prescribing antibiotics, rather he is saying you need to drive a different car (or maybe stop driving altogether), move to a different place (cities are much more efficient from a CO2 standpoint), and make major changes in how you live your life. Yeah, I'd be inclined to ask questions!