Since 2004 I've written 2,365 posts on this here Church of the Churchless blog. That's a scarily large number, which testifies to my commitment to churchlessness after spending 35 years being firmly churched in an Indian guru-based form of religiosity.
During the past fourteen years I've talked a lot about why I no longer believe in God. I've put forth numerous reasons for my conversion to atheism. Here's a fairly brief description of five key reasons.
(1) Existence must always have existed. Most religions teach that God brought our universe into existence. But I've never heard any believer argue that God brought existence into existence. That's because God must have existed prior to bringing everything else into existence.
So it sure seems like existence must always have existed. Otherwise, how could anything exist, including God? This leads to a strong argument against God: if religious believers say that God has existed eternally, this means that existence has existed eternally.
Thus where is the need for God, since eternal existence, along with certain laws of existence necessary for further acts of creation, fills God's role without the need to posit a supernatural being?
(2) Religions and supernatural belief systems differ widely. If God or some other supernatural entity existed, one would think that there would be considerable agreement about his/her/its nature. But there isn't.
Yes, I'm familiar with ideas such as Huxley's Perennial Philosophy, since I used to argue myself that there was a common denominator to the world's spiritual belief systems. However, I now realize that I was doing my best to cram wildly different forms of belief into the peculiarly-shaped hole of the faith that I followed.
There are literally thousands of different forms of supernatural belief in the world. The major religions are just the tip of the iceberg, given the number of animistic, ancestor-based, and other types of spiritual systems scattered across our planet. So either God exists in a dizzying variety of forms, or God doesn't exist.
I'm going with "doesn't exist."
(3) The cosmos is vastly larger than needed for most religions. Christianity, along with Judaism and Islam, are examples of religions that were founded by a human being who was considered to be in some fashion "God in human form." Meaning, if not actually God, a messenger from God.
Problem is, there are hundreds of billions of stars in our Milky Way galaxy, and hundreds of billions of galaxies in the observable universe, which is a fraction of the cosmos (the known universe, plus at least that portion of the universe that has expanded beyond our light horizon, since space itself has expanded faster than light since the Big Bang).
The makes Earth utterly insignificant in the grand scheme of things. Yet many, if not most, religions would have us believe that happenings on Earth reflected God's plan for creation. Well, if this is the case, why is creation so much larger than needed if humanity was made in God's image and has a special place in God's consciousness?
(4) Those who supposedly know God aren't anything special. Some religions teach that God sent a prophet, savior, or other form of messenger to Earth a long time ago. Other religions teach that there are God-realized people living today.
Regardless, at best there is scant (I'd say none) evidence that those who supposedly have come to know God are any different from you and me. Sure, there are plenty of stories about miracles, divine powers, and such, but somehow these all occurred in the past.
Now that modern science has ways of assessing whether someone has supernatural abilities, the flow of miracles has dried up. Which leads me and other skeptics to wonder whether they ever existed in the past, other than in the gullible minds of believers.
(5) There's no demonstrable evidence that God exists. This is an overarching reason that encompasses some of what I've already said in this post. I realize that plenty of people, alive and dead, are convinced that they've had experiences that prove God or some other divine entity is real.
OK. There also are lots of people who claim to have experienced alien abductions, seen Big Foot, had visions of heaven after almost dying, and so on. It simply isn't possible to accept that every subjective experience reflects an objective reality. That way lies, if not madness, an inability to distinguish fact from fiction.
Along with almost every other atheist, I'm very much open to evidence of God. I've meditated virtually every day of my life for the past fifty years, or so. I've prayed, sat at the feet of a supposedly God-realized guru, contemplated the nature of existence, looked for divine signs inside and outside of myself.
If God exists, God is doing a heck of a job of keeping hidden. So I choose to consider that God doesn't exist.
There are lots of other web pages that discuss why someone doesn't believe in God. Here's one that I found to be nicely convincing.