In the case of Christianity specifically, I think it's creators have done a masterful job of designing it to be self perpetuating.
Like other religions, it has the aspect of indoctrination at an early age. The earlier the age the better...but right off the bat even the youngest children learn about submission when going to church. They learn to sit quietly, behave, and to repeat the words they're told to repeat even if they don't fully understand the meaning behind them yet. Religious symbols like the cross and Jesus himself, hands folded in prayer, the eucharist, etc....these are all undoubtedly making a strong impression on young minds.
Christianity almost immediately begins to create an "in- group" mentality from the start. Stories like the biblical flood, Sodom, etc paint a picture of a world that is being swallowed whole by evil men with their evil/unchristian beliefs. You're told repeatedly that you don't want to be a part of that world...you want to focus on god and be a part of this in-group. You're told that anything good in the world comes from god...indeed, even the notion of good comes from god...so if you don't want to fall into the world of evil, you've got to stay in the group. These messages are repeated over and over many different ways until you believe that the only way to be good (to get to heaven) is to be part of the in- group.
To further reinforce this in- group mentality, tales of persecution and fear will get added to the overall message. The world will hate you, since its evil and you're good, and they will try to destroy you and the in- group. The message will be tailored to appeal to the people who are most vulnerable...the sick, poor, meek , humble, the persecuted. People who could use some help at a difficult time in their lives will be targeted by christianity for conversion. It's appealing to be told you're doing awful in life because you're good and the world is evil. The only reason your life was falling apart is because you weren't part of the in- group and without them...you're one of the evil people in the world.
This is the sort of message (and it's only part of that message) that ropes in the weak and vulnerable, keeps the indoctrinated in line, and perpetuates itself all at the same time. It's these elements, I believe, that are a big part of why religion is still around.