Low retention rate among atheists.

[Sheldon]

Why is Christianity's retention rate so high, and atheism's retention rate so low? Isn't that consistent with Christianity being true, and atheism false?

No, in the absence of any objective evidence your claim is simply a bare appeal to numbers, an argumentum ad populum fallacy, this is poor or weak reasoning. Assuming your assertions and stats were true, since you offered no citations, there might be any number of reasons or variables to account for it, I also wonder how many countries did this research examine, and how much power or influence does theism have in those countries, and how are atheists treated? In the US for example a great deal of bigotry and prejudice is still aimed at anyone who dare publicly declare themselves an atheist. 

It seems that the majority of people raised as atheists, upon becoming adults, leave "the faith",

Atheism is not a faith, it is defined in its broadest sense (the dictionary) as the lack or absence of belief in a deity or deities. FYI faith (religious faith) is defined as "strong belief in the doctrines of a religion, based on spiritual conviction rather than proof", that is the antithesis of atheism. Start by demonstrating that a deity is even possible, by supporting this idea with sufficient objective evidence, until you can do at least that much, atheism and atheists need do nothing, as the "burden of proof" lies solely and completely with those making a claim.