One thing that often crops up with Creationists talking about the Big Bang is the straw man that atheism requires a belief that “something came from nothing.” I’m not a cosmologist. I’m not an astrophysicist. I am merely an interested layman who tries to pay attention to what the scientists are speculating about the origin of the universe as we know it. Where did the universe come from? What caused the Big Bang? My honest answer is “I don’t know.”
Our understanding of the universe is incomplete. There are gaps we would love to fill, but don’t have the necessary evidence or understanding to fill them. Because I don’t know, I avoid recklessly filling them with false or untestable explanations. That’s something Creationists can’t seem to wrap their heads around.
Given the existing evidence, we can infer that the universe was once compacted into a singularity, and that includes time and space. Scientists have the gist of the universe’s history down to within some tiny fraction of a second after the inferred expansion started. The models we have right now break down if you try to work back any farther. We’ve got various hypotheses that are being entertained, but no one can honestly be confident in any of them because they haven’t been tested, yet. Some of these speculations haven’t been developed enough to conceive of a test. Until the test results come in, I have no reason to favor one over another, so I can simply say “I don’t know.” Because I don’t know of any evidence that favors any of those origin hypotheses, I lack a belief in any of them.
Here’s where things may get interesting: I think it’s possible that a god of some kind might have been involved, but I have no reason to think it’s probable. I don’t know of any evidence to support any god, so I lack belief in gods. Given the past failures of previous hypotheses about human-like supernatural beings, and the complexity they would add unnecessarily, I’d say they’re less probable than other origin hypotheses. That’s what makes me an agnostic atheist.