We don't know what came before the Big Bang, or if there was anything before it. It's also possible that there was no "before" because space-time as we know it may not have existed prior to the Big Bang.

To me, it's just a difference in mindset. Science recognizes the limits of what we know, tries to quantify what we know we don't know, and seeks to learn and understand the things we don't yet know. Religion often uses "God" as a catch-all to explain everything we don't know. To paraphrase NDT, this makes God an ever-receding pocket of ignorance. I'm ok with that.

What I'm not ok with is dismissing scientific findings in favor of accepting a storybook explanation on faith alone. One of the key tenets of the scientific process is that your hypothesis must be disprovable. The requirement to accept things on faith is antithetical to that idea, you can never disprove the existence of God. That alone makes the case for God's existence extremely weak to me. We can however disprove the creation story in Genesis if we're examining it literally. It's easily provable that the universe existed for more than a handful of days prior to humans. It's provable that life on earth existed for millions of years prior to humans. It's provable that light existed in the universe for billions of years prior to the existence of earth, life, and humans.

I don't take much issue with people holding their own personal religious beliefs, but I don't think it should override what we can observe about the world by hard evidence. I think religious texts that reference creation or history better serve as metaphors or parables than literal truth. It does also make me uncomfortable that children raised in religion are often taught to accept these stories as literal fact on faith alone- I think it hinders critical thinking and curiosity, and leads to a populace that is gullible and prone to accepting false narratives in ways that are more directly damaging to society.

I may have digressed a bit there, but to directly answer your question, I "believe" in the Big Bang to the extent that it is the best explanation of the earliest point in the existence of our universe based on our current knowledge. I am open to, and even expect within my lifetime, that the theory of the Big Bang may be modified as we gain further knowledge, or even thrown out completely if we are presented with contrary information. That's how we examine the world scientifically, my "belief" in the Big Bang is not unshakeable and I don't see changing my views based on new information as some kind of assault on my beliefs or admission of defeat. Rather, any new information that refines or changes our understanding is a victory. If God descends from the heavens tomorrow and publicly states that he created the universe exactly as is detailed in Genesis, and that all of the contrary evidence was falsely put in place as some kind of test of faith, I will readily accept it. I just feel the need to clarify "belief" in things I can observe, measure, touch, etc from "belief" in the context of taking something entirely on faith with no testable evidence. Have you ever heard of the idea of "Last Thursdayism?" It's sort of a thought experiment, that the entire universe and everything within it came into existence last Thursday, and everything you know of the time before that is just false memory you were created with. It seems silly on its face, but there's no way for you to disprove it. How is that any less valid an origin story than Genesis? Because it wasn't written on some old scrolls in the desert? How do you even know the scrolls or the desert existed before last Thursday? You could test the age of the scrolls with radiocarbon dating, but that method can also be used to disprove the Genesis story. You could present geological evidence that the desert is more than a week old, but I can present geological evidence that the earth is older than 6,000 or 12,000 or however many years old the Bible says the earth is this week. I'm not trying to mock your beliefs, just attempting to demonstrate that fairly comparing the theory of the Big Bang to a religious creation story is impossible, it's apples and oranges. You can compare the creation story of one religion to another, or even to any made up idea like Last Thursdayism, but even then you'll find it difficult to make realistic conclusions about which is more valid. Are we basing it on who wrote it? How long ago it was written? How many people believe it? None of these measures really indicate that one is more or less valid than the next.