How did Christianity become so popular

[SteveII]

Neither Christianity nor Islam is shrinking nor projected to shrink. In fact, atheism is projected to shrink (as a % of world population). 

http://www.pewforum.org/2015/04/02/relig...2010-2050/

Many Christians are baptized by the church so they are technically Christian but many do not feel like Christians and never go to church. There are Christians that do not truly adhere to the faith. Furthermore, many Muslims are leaving Islam but afraid to renounce it publicly due to fear of being put to death. Atheism is growing in Saudi Arabia, Pakistan and so on, although a number cannot be estimated.

We are reminded that the hurry in this matter has severe consequences in this world and the hereafter. The first is that whosoever is sentenced for apostasy  / renouncing Islam must be killed by the Muslim ruler.

Secondly: His marriage is revoked and his wife becomes forbidden for him.

Thirdly: His worship and good deeds become worthless.

The Messenger (s.a.w.) said: “Whoever changes his religion, then kill him” — Reported in Bukhari (Silsilah al-‘imaan wa al-kufr 25/2 — Muhammad Ahmad Isma’il al-Muqaddam)

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. 

• 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.[7]
  
  
• 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.[40]
  
  
• 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][46]
  
  
• It's been also reported that conversion into Christianity is significantly increasing among Korean,[47] Chinese,[48] and Japanese in the United States.[49] 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][52]
  
  
• Due to conversion, Christianity has grown in South Korea, from 2.0% in 1945[53] to 29.3% in 2010.[51]
  

https://en.wikipedia.org/wiki/Christian_...Conversion