Religions do need to bring in outsiders, if you want to call them that, but I think the vast majority will rely on future generations of their congregation they have at the moment to keep the numbers up. In a lot of churches you'll often find a few generations of families in there as the belief is taught to them and passed on at an early age.
Believers are all called to spread the gospel. We actually have the truth, and way to eternal life (Christians). So to hold it to ourselves would be unloving. That's why you don't see any false religions preaching, because they don't have the truth. The truth (Jesus Christ) convicts the conscience of people and that's why they find it uncomfortable.
This is probably the most accurate answer. I mean when you boil it down, MOST things are about money. Reminds me of a great book I read once, the Ascent of Money by Nial Ferguson.
it's normal for humans to advocate their beliefs and thoughts because humans are social creatures and part of that is to recruit. and having like minded people around you is comforting.
It's been recorded throughout time you know that religions are the most viable source of business in any civilization. Money is power, the faith and loyalty of your followers is power. I have seen religious blocks supporting one political candidate. I have seen religious cults buy vast amounts of lands to create sick communes with their followers. I've seen how influential a charismatic leader can get when he makes speeches-- advertises really, his beliefs. They need to advertise because more followers creates legitimacy and legitimacy makes money.
These days religion needs all the publicity it can get; the Internet is slowly killing it and extremists are giving it extremely bad PR.
It might be for monetary reasons for some, for others it might simply be because they have that belief and faith in their religion so much. For instance, my former classmate is a 7th Day Adventist member and she constantly talks about her faith. Will recruiting us get her money? No, not necessarily, it's just she believes in her religion so much that she wants others to see what it's like to be there as well.
My best friend told me his 1 experience, we both are big fans of spiritualism. OSHO commune is in my city, He goes there for meditation. once he was coming back, 1 person asked him, Are you .... ? do you follow ....... ? ( Dots are name of religion) he said, 'no' . then that person said with hatred, You'll go to hell. My friend answered in polite and gentle words. I don't have problem to stay in hell. If people like you are not there. whom to respect and whom to follow, this is individual decision of every human. If 1 religion have less but true followers, that's better than millions fake followers.
The flaw I see in this is that it's preaching and is like brainwashing. If people wanted to know surely they can go and discover it themselves without being approached? This harks back to the Puritan and Quaker days when each were trying to get people to follow their religion. I was on another forum where a person was a Scientologist and all he could do was spout not his beliefs, but theirs. Even though I am spiritual, I still have my own set of beliefs, which are a combination of what works for me.