I know there is no god- just like I know there is no ghost or there is no use for magic. There is no being or phenomena outside the scope of natural law. We may have cognitive limitations and we may not be able to explain everything in this universe. But the knowledge of mine is adequate and full; I don’t have any doubt. I had gone through the theology of religions like Christianity, Islam and Judaism-the chief monotheistic religions--and I have found no reasonable ground for thinking there is an Almighty who created humans, protects and takes care of them.
Yahweh was the tribal god of the Israelite tribe who later became their only god. There are differences of opinion as to how a tribal deity evolved into the Almighty. This god of Judaism was later adopted as the Supreme Being of Christians and still later by Muslims.
The reward and punishment which the respective gods have advocated as reasons for creating humans are simply nonsense. Why does god create children- half of all men and women have died before reaching adulthood- without attaining the ability to merit heaven or hell. I have also known Eastern religions like Hinduism & Buddhism and there is no evidence for a god there too. Hindus have deified nature phenomena and believes even earth, snake, tree, fire et all are gods. It is very easy to see how people came to believe all these. Buddhism and Jainism and even Confucians do not have the concept of god in the real sense of the term.
I am an atheist since I was 19 and my beliefs are well-thought out, weighed and solidified. Having born in a catholic family and having undergone training for Catholic priesthood I can vouchsafe there is no god whatsoever. As Stephen Hawking has said there is no need for a god to explain this universe. As Einstein has said there is no soul and once our brain stops functioning everything is over in a blink, our consciousness is shut off forever.
The difference between a believer of a particular religion and another is much more than the difference between an atheist and a theist. Muslims, Hindus, Jews, Buddhists, Jains, Taoists, Confucians -all- do not believe Jesus was god or that he came down, got crucified to atone for the sins of man. All religious believers other than Muslims do not think that Quran was directly dictated by god. Protestants disagree with Catholics about the virginity of Mary. Hindus think there are many gods unlike monotheistic Muslims, Christians and Jews. Except Christians and Muslims no one thinks of a heaven or hell. The ultimate goal of life for Hindus is emancipation from the cycle of birth and rebirth and for Buddhists it is Nirvana.
Each community believes what they do, not because it is the objective truth but because most people don’t have reasons beyond being comfortable with one community rather than another.