On one of the forums I frequent, the comments discussion drifted somehow to which religion had the most members globally. Someone tried to say it was Hindu/Buddhism and I of course had to correct him. Christianity is first with Islam a close second. I then made an off-hand comment that we all know why Christianity dominates and it's not because it's the one true religion. One of the theists took umbrage with this remark and wanted to know my feelings as to why this is true. I poked a little fun at him by asking how he didn't know why his own religion was the most popular, and he just up and left the discussion before I could offer my answer. So here is what I would have said to him:
The reasons are plenty and these aren't in any particular order of importance:
Convenience: If you want to be a Jew, there's the hassle of learning Hebrew, there's a dress code, you must attend Temple services, the food you can't eat is tasty and you must deal with bigotry.
If you want to be a Muslim, you must go to Mecca, pray at least five times a day, you can't eat certain foods, if you're a woman you must live your life covered from head to toe while living in fear as a subservient human. Plus there's the whole jihad bullshit instilled in Muslims.
Other religions have the same or similar restrictions on lifestyle that adds up to one huge inconvenience. But in Christianity? You don't have to do anything and you can still call yourself a Christian. Just ask for forgiveness while you watch your favorite reality shows featuring Real Housewives. Do whatever you want, just as long as you make an attempt to be a good person. You can do this in perpetuity. And don't worry, if you fail, just ask for forgiveness and get a clean slate.
One Christian group can accept evolution (Catholics) while another can dismiss it as the devil's handiwork (Young Earth Creationists), yet both remain Christian and both are considered correct. One can hate homosexuals, one can accept them because Jesus was loving, and both are correct. The bible is figurative, the bible is literal, doesn't matter, you're Christian. Want to pray today? Go ahead. Don't want to? That is OK, too.
Power: Christianity had/has the most power behind it, including weaponry and hierarchy (think Constantine, Crusades, Inquisition). Why are there so many Christians? If you didn't convert to Christianity, you were killed. Simple math. People believed or they died. And as families continued to raise their children that way, the numbers grew exponentially. You kill those that don't believe and you nurture and raise those that do. Again, simple math. It is easy to outnumber other religions when you are killing everyone who wasn't like you.
Money and missionaries: Christianity preys on the needy. "We will build a hut for you, but first let's gather round and discuss the miracles of Jesus. Are you hungry? We'll give you that sandwich just as I tell you the good news."
How many religions send missionaries out into the world to convert non-believers? Christians try to convert every non-believer or non-Christian they know. It's considered their duty. The other religions don't do that. It's easy to outnumber people when you have recruiters all around the world promising you stuff for your conversion, especially if you're desperate.
Christianity craves money more than any other religion, easily. How many times has a Jew knocked on your door to ask if you have a minute to talk about Yahweh? Any Muslims ringing your bell for a chance to discuss Mohammed? Maybe a Hindu has approached you to discuss Shiva. No? You don't say. Christianity loves money, it needs it, and in America it gets tax breaks so the more money they get, the more they keep. These people aren't trying to convert others for love and peace, they are being forced out to increase the flock, to increase the cash flow. And don't even think about bringing up charity, because that still ultimately benefits Christianity, and you don't need religion to be charitable.
So, you take all of these factors and it adds up to Christianity outnumbering all religions, but even with all of these reasons, Christianity is still dying and won't be on top for long. A recent PEW study shows Islam will overtake Christianity in the next few decades and non-believers will catch up as well. So, it turns out that even the threat of death and the promise of eternal bliss isn't enough to keep Jesus lovers in first place.