In our increasingly secular world, many atheists, agnostics, and hesitant believers alike have asked the age-old question: what reasons do I have to believe that God does or does not exist? In the following, let's consider just that.
First, let's consider what sort of 'reasons' ought be applicable to our hypothetical belief in God. Imagine I invite you over to my backyard, and show you my pear tree. I assert "This pear tree exists," and you will probably agree with me. The tree appears to be a pear tree, and it also exists in perceivable space, such that you can see and touch it. Then, my dual-assertion that 'this thing I am pointing at is a pear tree and this thing that I am pointing at also exists,' passes with flying colors.
However, after pointing out the tree, I mix things up by also pointing at nothing in particular, and saying 'this air exists.' Now, you must ask yourself the same questions as earlier: "Is that stuff air?" and "Does that stuff really exist?" Again, you will probably conclude that I am telling the truth. This time, even though you cannot see the air, or touch it, it may touch you in a particular gust, and it may blow some leaves on a tree, or what have you. Then, while we can't necessarily confirm a single body we could point to and say 'this is air, i can touch it and see it,' we can still be justified in believing that air exists because of the effects we perceive of air when air interacts with the rest of the world.
The extrapolation is apparent: God also lacks a concrete body, so far as humanity can see. Then, the key to ascertaining God's existence lies in perceiving God's effects on the world. Perhaps you believe that God heals the sick through prayer, or perhaps you believe that God caused you to meet your husband or wife, or perhaps you believe that God caused you to look down and find that twenty dollars on your lunch break. Does God exist, then? These would be your reasons to believe in God: you believe you have perceived his effects on the world, and nothing could affect the world without also existing.
However, as another thought experiment, after pointing at the air I say to you "You know, the reason the air moves is because of small, undetectable supernatural creatures constantly flying about, pushing the air for sport and play." You may retort, "No, I believe the cause to be quite clearly changes in atmospheric pressure." Which one of us would be right?
Human pragmatism has led us to prefer the explanation which grants us the greatest utility: if we believe in my supernatural sky creatures, for example, predicting rain may be a matter of dowsing rods and divination. However, if we believe in your theory of changes in atmospheric pressure, predicting rain belongs to the field of meteorology, which is generally more accurate.
Applying this chain of thinking to the existence of God is tricky, though. So far as the eye can see, there is not a single event that can be unambiguously attributed to God. The modern age is notoriously lacking in burning bushes, pillars of salt, fire, and brimstone. As a result, God's only remaining avenue of interaction with the world is indirect: God can cause you to look down and notice money, or God can tweak things here and there so that you meet a significant other that brings you lifelong bliss. Of course, these things can very easily have alternate explanations, as well.
And the crux of the issue is exactly that: there is no way to be sure that God exists, since every single 'act of God' has a potential physical explanation, but it's also impossible to be sure that God does not exist, since God could be causing those physical explanations by tweaking outcomes here and there in the mess of randomness we humans still don't understand. The issue is left to the reader, then. Does God exist?