If the Pope tells people not to drink water, despite the fact that they are thirsty, not many of them are going to obey him, no matter how indoctrinated they are.

If he tells them that their local water is unhealthy, and that they should filter it before drinking it, a fair share of them will probably take his advice.

If the local people have very limited education, and the Pope tells them that filtering their water before drinking it will increase their risk of deadly illness, a fair share of them will probably take his advice.