One of the most basic techniques used by extremist cults to keep their followers in line is to isolate them from the rest of the society. The less they have contact with outsiders, the better.
The largest religions can't afford to do this for practical reasons. They need to seek a proper balance between keeping followers in line and having a working society (because without a working society both would crumble, so it wouldn't be a tenable system). Thus followers are allowed to be in contact and socialize with "outsiders". However, a form of isolation is nevertheless imposed on them. It could be called "soft isolation".
Rather than isolating them physically, they are isolated mentally. This is especially true for the so-called charismatic denominations. This is achieved in the form of propaganda and what amounts to conditioning and brainwashing: The members of the religion are taught again and again how to handle questions and opposition to the religion, as well as "worldly" behavior and attitudes shown by outsiders. They are told again and again things like "when an outsider says this, he really means this", and all kinds of normal behavior are labeled as sinful and detestable, and it is strongly emphasized that separating oneself from such "worldly" activities is a virtue one should aim for. Often even the same type of activities performed by both groups (such as partying) is seen as detestable when done by outsiders, but acceptable when done among other believers. (Outsiders only participate in parties with sinful goals and reasons, while believers do so in order to praise God or whatever).
Separation from the world at a conceptual level is one of the key tenets of many denominations.
It is also emphasized that believers should regularly attend church services and activities. Big warnings are given that failing to do so may tempt believers to leave the religion and embrace worldly sins, and that many have been lost when they have stopped going to church and socializing with other believers. The reason for emphasizing the importance of this is, of course, that a person needs to be re-affirmed the key tenets of the belief system in a regular basis, as that stifles doubt and skepticism.