How did Christianity become so popular

[SteveII]

Perhaps, but that is taken into account in the projections. In addition, nothing you pointed out means that all these nominal Christians or Muslims don't believe in God. They can believe in God and not participate or like organized religion. 

The Christian church continues to grow freely. I posted this in another thread recently. 

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". 

According to the World Christian Encyclopedia, approximately 2.7 million converting to Christianity annually from another religion, World Christian Encyclopedia also cited that Christianity rank at first place in net gains through religious conversion.

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. 

Studies estimate significantly more people have converted from Islam to Christianity in the 21st century than at any other point in Islamic history.[8] Conversion into Christianity have also been well documented, and reports estimate that hundreds of thousands of Muslims convert to Christianity annually, significant numbers of Muslims converts to Christianity can be found in Afghanistan, Albania,[9] Azerbaijan,[10][11] Algeria,[12] Belgium,[13] Bulgaria,[14][15] France,[16] Germany,[17] Indonesia,[18] Iran,[19][20][21][22] Kazakhstan,[23]Kyrgyzstan,[24] Malaysia,[25] Morocco,[26][27] Netherlands,[28] Russia,[29] Saudi Arabia,[30] Tunisia,[31] Turkey,[32][33][34][35] Kosovo,[36] The United States[37] and Central Asia etc.[38][39] Many of the Muslims who convert to Christianity faces social rejection or imprisonment and sometimes murder or penalty, for becoming Christians.

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. 

Data from the Pew Research Center that as of 2013, about 1.6 million adult American Jews identify themselves as Christians, most are Protestant.[41][42][43] According to same data most of the Jews who identify themselves as some sort of Christian (1.6 million) were raised as Jews or are Jews by ancestry.[42] Data from 2013, show that 64,000 Argentine Jews identify themselves as Christians.[44] According to 2012 study 17% of Jews in Russia identify themselves as Christians.[45]

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. 

It's been also reported that conversion into Christianity is significantly increasing among Korean,[47] Chinese,[48] and Japanese in the United States. In 2012, the percentage of Christians of these communities were 71%, more than 30% and 37% respectively. [50]

• Due to conversion, the number of Chinese Christians has increased significantly; from 4 million before 1949 to 67 million in 2010.[51]
  
  
• Due to conversion, Christianity has grown in South Korea, from 2.0% in 1945[53] to 29.3% in 2010.
  

[url=https://en.wikipedia.org/wiki/Christian_population_growth#cite_note-projects.pewforum.org-50][/url]

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".