It's all semantics. People believed the earth was round well before we actually got an astronaut in space. At least a teeny, tiny part of us could have doubted it until we saw the pictures. It is certain enough, though, to apply to our daily lives. Which brings me to my next point..
You really don't. There are experts in philosophy, geology, physics, chemistry, mathematics, etc. that are Christians, Islamic, Hindu, etc. Complete experts in their fields--the very STEM fields that try so hard to argue for Atheism--and they still maintain their faith. There is still not enough evidence to completely discount the religion itself, albeit many discount the WAY it is taught and the translations used.
At the end of the day, we don't know, and we never will. That's the "We agree to disagree" argument ending statement for religion vs atheism. And that statement really means that no one can know with certainty who is right. We can only know practical certainty to use within our daily lives until such a time as one dies. No one knows everything. For Christians, that certainty is within God. Only he knows, and it is the only certainty in life. And really, it is not a bad practical application in and of itself. Humans are usually the ones that make it more annoying than it has to be.
I have to say though, for atheists to rely so heavily on science, they are WAY too certain that everything is not connected by something. At the end of the day, if an atheist does not believe that no proof is not a sign of non-existence, they are being just as blind as the Christians can be.