If you had to reduce why you believe or lack belief of a God in general, what would you say? This doesn’t have to be your full rational answer on how you might convince someone else that your opinion is a reasonable one to hold, but more so the gut reactions and minor realizations that lead you to this opinion.
Briefly, I want to approach this from an epistemological angle—especially since you’re a philosophy major. I have all sorts of reasons to not believe in any god, but on epistemology, there is no viable way to attain true belief in a god—let alone knowledge. Neglecting that true beliefs can be held mistakenly (i.e. I perceive and thus believe that the pavement is curved and moving; though my perception is roughly correct and though I hold a true belief, closer inspection will show that my eyes are being deceived by the Summer haze on the pavement and thus, what I hold as a true belief is actually not true), a true belief arises from many sources—one of which is perception. You perceive that you’re in a stadium full of fans. Looking down at the stadium, you see umpires, bases, dugouts, etc. Thus, from perception you then believe that you’re in a baseball stadium. This is a true belief that will be corroborated by everyone who has ever attended a baseball game. It also qualifies as knowledge.
Whether you’re a theist, deist, pantheist, panentheist, or something else entirely, true belief is inaccessible. Traditionally, gods cannot be seen. If something doesn’t meet our vision in any meaningful sense, that’s already a strike against it. One might argue that that which we can’t see can still leads to true belief, knowledge, and justification. That would be correct. Gravity, though invisible, can be accessed via its effects. Though gravity isn’t palpable, its effects in the universe are. So it turns out it can be seen but indirectly so. So a god could meet our senses in this respect. But not even in this sense does a god meet our senses. Gods, especially in different forms of theism, reside in the psychological and emotional. People speak of hearing gods in their heads or feeling their presences or being given visions and/or dreams. One is welcome to believe these so called revelations, but still, there’s no way to transfer such mere belief to true belief—let alone, I reiterate, knowledge.
But one may argue that there are other ways of knowing (i.e. testimony, a priori). I’m still a little shaky on where I stand with the a priori/a posterioridistinction, but a priori dictates that if you know that Ken is shorter than Ben, then Ben is taller than Ken. I myself am confused by how this qualifies as a priori or intuitive knowledge (especially given its definition, from the prior). In any event, testimony is reducible to perception, experience, and so on and thus, reduces to the first person or to the introspective. A priori, in a sense, also reduces to these things. If you know a bald eagle has a greater wingspan than a sparrow, then you also know that sparrows have shorter wingspans than bald eagles. But what equivalent measure is there for deities? How can we arrive, a priori, at a mere belief in a god? How do we then get at a true belief? How then do we arrive at knowledge—“I know this god exists.”
Ultimately, putting all of the philosophical jargon aside, belief in a god rests on faith. Faith, crudely defined, is belief without knowledge. Some ardent Christians and Muslims may boldly proclaim to know that god exists, but how many have justified this knowledge? My tally rests at zero. The pretended knowledge of the devout has never been justified; in my book, belief in a god isn’t a true belief. It is faith-based speculation; in the absence of justified knowledge, there’s only conjecture. So that’s just one philosophical angle I can use to explain why I don’t believe in gods. I can approach this also from an ethical point of view, but that would be much too focused; I would only be able to rule out those deities which are supposed to interact with the world. Arguments from ethics will not rule out deistic or pantheistic concepts because nothing usually requires such gods to interact with the world in any meaningful sense.*
In a nutshell, belief in a god doesn’t qualify as true belief. It also doesn’t qualify as knowledge. Even when it is claimed to be knowledge, that knowledge isn’t justified in any cogent sense.
*From a more empirical point of view, how can we arrive at true belief, knowledge, or justified knowledge, if what we merely believe in lies outside of what we consider reality? Anything considered outside of reality (whether gods or Plato’s Forms and Ideas) are inaccessible in any of the aforementioned ways. True belief, knowledge, and justified knowledge cannot derive from anything that transcends reality—especially in cases where there’s no interaction between said transcendence and our reality. This renders deism and panentheism as epistemologically incoherent.