But few beliefs involve such simple and objective subjects. You are picking a very biased example.
The real subject of this conversation is belief in deity. And that is not even remotely so simple. Almost no one believes in deity absolutely. Some people claim that they do, but in truth it's very likely that they have their doubts, too, but are just choosing to ignore those doubts, and deny them to us. And they may be aware of their doing this or they may not be.
The point, though, is that when some degree of doubt is present, which is almost always the case, then we have a choice whether or not we will acknowledge that doubt, and to what degree we will acknowledge it. And the result of that choice will effect the way we perceive the world around us. It will effect what we recognize as "evidence" for or against our believing.
You will notice that those who claim that they believe in deity absolutely, also tend not to recognize any evidence contrary to their chosen belief. And the same goes for those who claim that they absolutely do NOT believe in deity. They, too, will tend to ignore and deny any evidence that might suggest that their chosen belief is wrong. And even among those who accept their own doubts and confusion regarding the existence of deity, will tend to be biased in favor of their preconceptions.
What we see in the world around us is very often determined by the way we choose to look at that world. And in this way, we tend to see the "evidence" that we set ourselves up to see, and we tend to experience the world in the way that we set ourselves up to experience it, and therefor to believe about the world what we have chosen to believe by the evidence and experiences we've created for ourselves. And again, we may be aware that we're doing this, or we may not be. Very often we are not aware of it.