What Buddhism Does and Does Not Believe In
And just to be fair, a good Buddhist denies his/her own existence as well. This strikes Westerners and monotheists as contradictory from a rational perspective, which it is. But Buddhism believes that Ultimate Reality lies beyond rational human thought's ability to comprehend.
Let me explain Buddhism in two steps. In the first, Buddhism developed from Hinduism, taking many beliefs from it. One of these was that the natural and supernatural realms, as Western religions would term them, belong to a single structure called Samsara. Everyone and everything living in Samsara is subject to reincarnation -- upon death they are reborn into Samsara. Again and again and again. This applies to the gods in the heavens as well as to the humans and animals on Earth. The gods may be strong, spiritual beings who live long lives, but they do not live forever and they are not all powerful.
The most powerful beings are humans who have achieved enlightenment. In the Sanskrit language, enlightenment is called bodhi, and one who achieves it is a Buddha. In Mahayana Buddhism, such enlightened beings have two possibilities, they can either pass on into Ultimate Reality and end their ever-repeating existence in Samsara, or they can use the power they gain through enlightenment to help others. These are known as bodhisattvas, "enlightened saviors." Bodhisattvas wield their vast power to assist others, usually relieving their suffering or helping them to achieve enlightenment.
What is enlightenment?
The answer brings us to the second step of the explanation. According to Buddhist belief, humans live lives of suffering and loss; pleasure and enjoyment occur rarely and fleetingly. Reincarnation brings nothing therefore but repeated and continuing suffering. Most Buddhists live such difficult lives providing for themselves and their loved ones, it is believed, that they have little choice but to request help from gods and the more powerful bodhisattvas and buddhas for help.
But a few Buddhists eschew ties to family and others to enter a monastery and seek enlightenment. Enlightenment enables one to escape the recurring suffering of life in Samsara by realizing that all Samsara is an illusion. Ultimate Reality lies beyond it.
Humans are deceived by the apparent reality of the world and universe in which they live. They become attached to it and to the people who inhabit it, and to themselves. They love their parents, spouses and children. They hate their enemies. They become proud of their talents and skills. They become vain about their looks and their social status. They desire comfort, sufficient food and possessions. These emotions attach them firmly to this world and its reality.
This is all an illusion; it is not real, despite what it seems. When monks enter a monastery, the first thing they begin to learn is that one's attachment to people and things gives (seeming) permanence and solidity to the illusion of life. They learn detachment, to remove the links that tie them to people and objects in Samsara, the links that give Samara the appearance of reality.
Once this is mastered, monks must learn that they themselves are not real; they are part of the illusion as well. This realization is extremely difficult, given the persistence of each person's ego, and may take two or three reincarnations to achieve. But once enlightenment is found, then the illusion that the world, the heavens, and their human and divine inhabitants are real falls away. One knows Ultimate Reality as it truly is. Is this difficult? You bet. Do many Buddhists achieve enlightenment? Only a few. But in Buddhist belief, that does not make it any less certain.
If Buddhism, with its denial of what Douglas Adams calls "life, the universe and everything," still qualifies as a religion, then atheism's denial of god(s) poses no obstacle to its classification as a religion.