What is atheism?

My answer on Quora.com to the question

How can someone be an atheist if they have not studied all the religions?

That’s a good question. By the same reckoning, how can anyone say his/her religion is the right one until he/she has studied all other religions?

The fundamental idea behind most (except Buddhism) is that there is a god; that god created man wholly and as you see him today in his image (though Hindus occasionally come out with the theory that god is formless, emotionless and without attributes, but nonetheless worship gods that are ornamented with gold, decked with flowers and jewels and armed with primitive weapons in their multiple arms), that he/she sits in judgment of human actions, and most importantly, no matter what crimes one might have committed, one can be forgiven and redeemed simply by approaching this god or his agent with prescribed rituals, bodily exercises (bowing, kneeling, prostrating, holding one’s ears etc.,) and some ancient phrases called prayers. Only minor nuances vary from one religion to another.

The atheist denies those core principles. Since Buddha said that the universe follows certain laws which need no supervision – which is what Stephen Hawking has been saying 2516 years later – you might say that Buddhism, essentially, is atheistic. It’s another matter that some Buddhists have made Buddha himself their god and thereby lost the status of atheism.