Christians sometimes claim to be certain about spiritual matters. This can be everyday things like, "I know this new job is right where God wants me," or more important issues like, "I know the Bible is the word of God," or, "I know Jesus is the Son of God."
But Christians do not have sure knowledge of these things. They believe them -- deeply and sincerely, and for all sorts of reasons -- but they do not know them in the same way that we know that fire will reduce a book to ashes, that there are billions of galaxies in the universe, or that gravity works. Some Christians claim this kind of knowledge, but they are wrong.
The same goes for Christians -- and any religious person -- who would say, "I know God exists." No one can know that God exists in the sense of proof or logical demonstration. Rather, people of faith believe God exists for all sorts of reasons that can't be laid out in a spreadsheet or observed through a telescope.
Atheists are in exactly the same boat.
What holds true for religious people when they talk about God holds for atheists when they talk about not-God.
Some atheists claim to have a sure and certain knowledge about spiritual things. "I know -- through reason, logic, and evidence -- that God does not exist." These atheists feel that their position is intellectually superior to a belief in God. God does not exist because what cannot be established through "reason, logic, or evidence" is not real.
This sounds rational and objective, but there is a lot of belief tucked away in this assertion. Atheists do not know God does not exist; they believe it.
To say that God's existence is detectable with certainty through reason, logic, and evidence is a belief because it makes some crucial assumptions. For one thing, it assumes that our intellectual faculties are the best, or only, ways of accessing God. This is an assumption that privileges Western ways of knowing and excludes other wholly human qualities like emotion and intuition.
It also reduces God to an object, a thing, a being among all other beings, whose existence is as open to rational inquiry as anything else. It is an old argument but a good one: any god worthy of the name is the source of all being, and therefore not one more being alongside all others subject to rational control. Any god like that isn't God at all.
People can think what they want about God. My point here is simply this: no one knows whether our intellectual faculties can determine with certainty whether there is a higher power, prime mover, or whatever you want to call god. That is a belief.
Also, all people, atheists included, believe worthwhile things for which there is no compelling evidence whatsoever. For example, many people -- scientists, philosophers -- believe in the principle of uniformity: what we observe now of the laws of nature happens everywhere in the universe, always has and always will.
I happen to believe this is true, but what I believe isn't the point here. The point is that there is no empirical evidence for this principle, nor can it be logically proven. In fact, there is no evidence for the principle at all unless we assume it to begin with.
Why do people accept the principle of uniformity? Because it can be used to construct coherent scientific explanations of the universe, and that is a good reason to accept it. But this is not too far from what religious people say about their faith. Religious beliefs can be used to construct coherent explanations for things like why there is something rather than nothing.
All of us accept as true ideas that seem to work well, that make sense of our reality. We do not know with certainty that they are true because of reason, logic, or evidence; we believe they are true because they work.
I know some real live atheists, and they do not claim to know as much as some others do. The reason that they are atheists is that "God is" is a less compelling proposition to explain their reality than "God is not."
They did not come to this sure and certain conclusion by a calm and logical assessment of the evidence (as opposed to the unreasonable and illogical faith of religious types). Rather, they came to their atheism for many different types of reasons, some of which are too subtle to quantify.
They do not claim to know that God does not exist; they believe it to be so because it makes most sense of their own lives and the world around them. This is not sure and certain knowledge; it is a belief.
Oddly, some Christian fundamentalists and some atheist fundamentalists suffer under the same delusion, that their view on ultimate reality is fully supported by reason, logic, and evidence.
Both are wrong.
For both the religious and atheists, there is mystery. Atheists are free to be atheists, but they don't know any more than anyone else.