Do you believe in the existence of a god?
No I don't, and this is why.
When I was very young, I saw there were many different varieties of religious beliefs. Some had one god, some had several. Each religion started in its own part of the world and despite their differences, believers all thought their particular religion was the one true version of reality. That began to make me suspicious that maybe they were all imaginary.
I since learned that religion is simply a normal part of human behavior, and belief in higher powers comes naturally to the human mind. There are other reasons as well that organized religion arose among all human cultures, though each developed its own unique dogma.
There have always been atheists, people who simply didn't have much attraction to or need for belief in gods. I appreciate how some religious believers can't imagine a universe without a supernatural creator or how someone could not believe in God. That's OK with me, I don't think you're stupid, even if you think I'm nuts.
So for me, there's enough physical evidence out there to explain how everything in the universe came to be (even the universe itself) without the need for a supernatural creator. That fact, plus knowing the workings of the human mind, made my mind conclude that belief in God made no sense. Each one of us is unique, and we're all on a continuous spectrum of belief in God that goes from none to strong. If we can all respect that fact, and each other, we should get along fine.