Why do Christians become Christians?

[SteveII]

http://www.pewresearch.org/fact-tank/201...ous-group/

The main reasons for Islam’s growth ultimately involve simple demographics. To begin with, Muslims have more children than members of the seven other major religious groups analyzed in the study. Each Muslim woman has an average of 3.1 children, significantly above the next-highest group (Christians at 2.7) and the average of all non-Muslims (2.3). In all major regions where there is a sizable Muslim population, Muslim fertility exceeds non-Muslim fertility.

The growth of the Muslim population also is helped by the fact that Muslims have the youngest median age (23 in 2010) of all major religious groups, seven years younger than the median age of non-Muslims (30). A larger share of Muslims will soon be at the point in their lives when people begin having children. This, combined with high fertility rates, will accelerate Muslim population growth.

Do you a link that shows data on adult conversions to Islam?

You are probably right--a bias will form. Is your assertion that faith is a direct result of locale and upbringing? That does not make sense if at least 15% of Christians (in the US) become so as adults nor does it explain the tremendous growth of Christianity in Africa and China (which would presumably be a much higher rate of adult conversion. 

It seem like more of an opinion rather than a fact that Christians should just "accept".

So Islam growth is caused by parents/indoctrination/locale but christians are a bit more discerning?

 A couple of points. 

1) It does not seem that Muslims evangelize anywhere near the level of Christians. Yet they are growing. Pew research indicates their projections are based on population growth.
2) Muslims are becoming Christians. While I could not read the whole paper, a well cited source from Wikipedia: According to 2015 Believers in Christ from a Muslim Background: A Global Census study estimates 10,283,700 Muslim convert to Christianity around the world (https://en.wikipedia.org/wiki/Christian_...ion_growth). The paper itself: https://www.academia.edu/16338087/Believ...bal_Census

Related to the OP, Muslims do not have the life/teachings of Jesus, which I claim resonates with a large number of people and is the main cause of adult conversion.