Most supernatural belief-systems seem have a need to have their followers to meet regularly and hear more and more about their beliefs often at weekly basis. At the same time agnostics, freethinkers or atheists do not seem to have any such need.
It is all about keeping up or even strengthening of the faith. When a human being adheres to a set of beliefs that is not based on any kind of observable facts, it is quite inevitable that, at some point, he or she can begin to worry about the real basis of these beliefs.

A extremely basic problem for all of the purely faith-based religions is how to prevent this kind of private thinking from taking hold in the minds of the people. A regular common strengthening of faith is a method that is successfully used to keep up the mental support for claims that cannot be verified in the real world.
A scientific fact is that when a person in a strong position of authority repeats anything often enough there will be a lot of people who will believe it without even thinking. If the person spreading these beliefs does, in fact, himself genuinely believe that his allegations are true, the effect can be even stronger.

Evolution has created a strong trend in humans towards supporting the authority in any human community, as the wellbeing of human communities have often depended on it. A preacher or priest can surprisingly easily achieve a similar position of authority as the real community leaders or the teachers do enjoy. This happens, even if the teachers are the real carriers of true common knowledge in modern societies.
However, a preacher can be just spreading ago-old and even harmful ideas that were born in an undeveloped semi-nomadic community thousands of years ago.

Baptism of Vladimir. -Wikipedia

For centuries, the weekly gatherings of the religious communities were also in the western world the only real source on information for many members of the society. No rival information what so ever was, in fact, all too often available.
Why was participating in these ceremonies obligatory and compulsory for centuries? This happened even if there was no real fear of competing information reaching these people. This is because the upholders of a belief-system must take into account the fact that individuals do still have normally working minds. This fact can have an effect even if when these people do not get any real contradictory information.
The regular strengthening of faith is also sorely needed. If people are left to their own devices they easily start comparing the physical reality around them and the claims made by their faith. During this process, it just is easy to slip outside the realms of faith, even when you are not given any contradictory evidence.

Faith-based religions have been so successful because the need to belong to a community is one of the strongest needs any human being has. For centuries, the longing for a sommunity was much stronger than the similarly quite natural desire to understand reality as it is.
All faiths are aware of this threat to their systems of beliefs. For this reason also so many of them want to gather their flocks for weekly moments of repeated indoctrination. They know that the battle against the normal human reasoning is an unending one and one that needs constant vigilance.

The creators and upholders of common belief-systems have not been evil people in general. They were just building systems that did guarantee the continuation of the belief-systems that they did love and respect.
Evolution has been at work here too. One must remember that in the constant battle for survival among the existing belief-systems, only those belief-systems have survived that have developed ways for controlling the thoughts of their followers in the long run.

When seen from the inside, these methods are of course just the most appropriate and effective ways for keeping up the religious faith as constant as possible. For the real believers, even the worst cases of indoctrination and brain-washing are just effective ways for spreading the Only True Word.

(This piece completely refurbished on 4th of March, 2012)