How did Christianity become so popular

[Brian37]

Maybe not all of them are atheists, but it could be that most of them are. Also, if they don't practice organized religion, The Church will lose followers. Whenever you like it or not, the church is the sourse of christianity, and without it people will eventually lose interest in religion.

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". 

So it's the religion with the most conversions. While there are atheistic religions, many atheists are nonreligious, so there are still chances that Atheism is growing more than Christianity.

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

Many muslims migrate to chrisitian countries, where their religion isn't seen as very good. That could be a reason why many of them convert to Christianity.

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. 

Christianity did originate from Judaism, and there are people from all countries and nationalities who change their religion. Many Jews also live in christian countries, so they might've been influenced by it. There are some christians that blame them for the death of Jesus after all.

Or...some Jews find Christianity more compelling. 

Same arguments as for muslims and jews, besides the fact that the people in these examples aren't as much discriminated against.

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".

We don't care Steve, big numbers in believing in old mythology does not make an invisible sky wizard real.

It merely means humans MOSTLY get passed the idea at birth prior to developing critical thinking skills.

Allah isn't real because of numbers, Yahweh isn't real because of numbers. 

I hate to burst your bubble buddy, but just like neither humans nor religion existed 4 billion years ago, religion nor humans will exist, in 5 billion years. I am sorry it bothers you to hear that you are finite, but there is no sky hero helping humans.