How did Christianity become so popular

[Der/die AtheistIn]

Dropoff rates are nothing new and are accounted for. Also, I think you are confusing your idea of "the Church" with a capital "C" with Christianity. It is the far less structured Protestant and independent end of the spectrum that is growing worldwide. There is no one organization that is responsible for this--so nothing for people to leave and certainly none of this leads to a conclusion that without some sort of capital "C" Church, "people will eventually lose interest in religion". 

Democracy needs rulers in order to exist. If the system was something else, there still could be people who identify themselved as democrates, but that would not change the fact the the country itself isn't.

But atheism is not and is not projected to even grow (as a % of population) --let along grow more than Christianity. 

There are very many in the closet atheists, Poles reflect what the majority say out loud. Also, atheism is more popular on the Internet than religion. 

That's really weak. How about the countries on the list that are predominately Muslim or have large Muslim populations? Family pressure would apply no matter where you moved. 

I didn't say that this is the only reason, or even the biggest reason. But if you want a better argument, you've helped me find it, thank you very much! Many muslims feel pressured to keep their faith and have seen the horrors that their religion is capable of, so they want to leave their faith. At the same time, they might still feel the need to follow a religion, because the belief in a god makes them happy, and since Christianity today isn't as dangerous as Islamism, and it's also a very big religion, they feel tempted to follow it.

Or...some Jews find Christianity more compelling. 

Again, I didn't generalize, I said some of them, not all of them. And about those who find "Christianity more compelling", for some of them it can be because Christianity is more popular, and humans usually search after what's popular, no matter the quality.

In all these cases, you are assigning alternate reasons other than the default  reason for a freely-chosen religious conversion: that individual finds the message compelling. You are guessing as to other's motive with no evidence and your reason seems to be "there must be another reason".

In all my examples, I didn't state that every single one of them has the exact same reasons, I didn't generalize. People can freely convert to religions for reasons other than a compelling message. And even if they find the message compelling, it doesn't mean that it truly is, people are and have been wrong about many things. Everyone, myself included, isn't flawless.