You shouldn't. They don't exist. Unless an atheist can prove that there is no God, then they simply don't know. We have a word for people like that - Agnostics.
Atheism is a belief system.
They believe there is no God, and that human beings are supreme on earth. They can't prove it, but they are sure of it. They aren't unlike Christians, in that Christians are justified by their faith as well.
I don't seek to convert atheists, because sadly, the logic of their positions are untenable. I have tried to get them to acknowlege that in the absence of proof of God's non-existance, there must then be even a scintilla of doubt. This doubt, no matter how small, places them in the category we know to be agnostics.
It was then I realized that atheism requires an enormous amount of faith.
It is impossible to prove a negative.