Since atheists know this, they are only TRYING to be atheists. And it will always be difficult for them to be atheists, at least until they have examined the whole universe to see if God doesn't exist somewhere.
Atheism is self-contradictory.
Ok so because it MIGHT exist we can't deny it?
So I guess we also have to believe in the unicorns because they might be on mars or wherever...
The simple fact of hearing about the suggestion of God, makes you to have an opinion about God in both ways:
1. belief in His existence;
2. belief that He does not exist.
Since you know that He might exist, means that you are not an atheist.
This doesn't stop you from denying on the outside, formally, into the public. It only shows what you really believe through all your denial.
There might be real unicorns somewhere in the history of the earth. They might not exist alive, now. But there may be fossils.