|Lightning in my front yard|
Although there are, relatively, a small percentage of atheists in the world, when your field of study and teaching is philosophy (esp. philosophy of religion) you tend to have some of them as students who seem interested in life's Big Questions, such as: Does God exist?
For the percentage of atheists in the world see Phil Zuckerman, "Atheism: Contemporary rates and Patterns," from the Cambridge Companion to Atheism. Zuckerman writes: "Determining what percentage of a given society believes in God – or doesn’t -- is fraught with methodological hurdles."
Correct. For example, most of the young self-designated atheists I have met have not been reasoned into their atheism, but rather are responding to the hypocrisy of their "Christian" parents or the church they were raised in. That is sad. That also presents an irrational argument for their atheism, which can be represented like this.
1. My Christian parents are hypocrites.
2. Therefore, God does not exist.
There is no claim of inference from 1 to 2. Premise 1 (P1) may be true, and if so it is sad. But one cannot logically conclude from the truth of P1 that the conclusion (C2) follows. Indeed, it clearly does not. To reject God or "disbelieve" in God because of P1 (or variations on P1) is irrational. Most of the young atheists I meet began with P1, and only later scoured the Internet looking for reasons to justify their leap of faith.
Behind P1 there may be a supplementary premise, which is: P1' - God didn't work for them (viz., their parents). But even with the addition of this, still one cannot arrive, via P1 and P1', at C2.
What does follow, from my vantage point, is that hypocrisy in the church today and in Christian parents is a significant reason for the turning of some young theists to atheism.