What if you ran a cult, and needed to cut off your followers from the constant flow of information from the outside world?
That outside information is potentially devastating to member loyalty. It could cut your income to shreds, and undermine your reputation in the hearts and minds of your followers. But coercion is not compatible with modern life, and as well-educated as your potential recruits tend to be, you will never be able to directly persuade them to ignore the rest of the world. You can't just tell them to immediately shut it off-- close their eyes and ears and bury their heads in the proverbial sand. They'll get suspicious. And such bold requirements would ring in their consciences as your critics and their non-cult family members constantly repeat it back to them, desperate to prevent recruits from joining you.
So, what if you could get them to do it it to themselves? What if you found the perfect doctrinal blend of fear and guilt that would create in your followers the overwhelming desire to voluntarily ignore all outside information? What if you could get them to fear-- truly fear, as in be phobic about-- such contact with the outside world? As if this (they may come up with great pejoratives for it like "worldliness") were a direct threat to their eternal salvation, and that of their children?
Go one better, in fact: what if you could get them to do all the persuading for you? What if they even convinced one another that the more they cut themselves off from non-cult sources of information, the more morally pure and superior they would become? As they competed with one another to be the most ignorant of the real world, you wouldn't even have to issue any instructions to do what they already enjoy doing-- becoming the most ignorant/pious believer in your cult! (This also leaves no paper trail for your critics to use against you, and leaves you with plausible deniability about this little gem of genius you stumbled upon).
Even further beyond! What if in so insulating themselves from everything "worldly", they became convinced that they were fulfilling a prophetic destiny critical to the conflict between good and evil throughout the ages? What if they wrapped this voluntary ignorance up inside the very meaning and purpose they had assigned to their lives, to your cult, and to its preservation?
Hallelujah! You've just established one of the fastest-growing religions in the world.