Atheism is traditionally defined as disbelief in the existence of God. As such, atheism involves active rejection of belief in the existence of God.

However, since there are many concepts of God and these concepts are usually rooted in some culture or tradition, atheism might be defined as the belief that a particular word used to refer to a particular god is a word that has no reference. Thus, there are as many different kinds of atheism as there are names of gods.

Some atheists may know of many gods and reject belief in the existence of all of them. Such a person might be called a polyatheist. But most people who consider themselves atheists probably mean that they do not believe in the existence of the local god. For example, most people who call themselves atheists in a culture where the Judeo-Christian-Islamic God (JCIG) dominates would mean, at the very least, that they deny that there is an Omnipotent and Omniscient Providential Personal Creator of the universe. On the other hand, people who believe in the JCIG would consider such denial tantamount to atheism. Baruch de Spinoza (1632-1677), for example, defined God as being identical to Nature and as a substance with infinite attributes. Many Jews and Christians considered him an atheist because he rejected both the traditional JCIG and the belief in personal immortality. Thomas Hobbes (1588-1679) was also considered an atheist because he believed that all substances are material and that God must therefore be material, not spiritual. Yet, neither Spinoza nor Hobbes called themselves atheists.

Epicurus did not call himself an atheist, either, but he rejected the concept of the gods popular in ancient Greece. The gods are perfect, he said. Therefore, they cannot be the imperfect beings depicted by Hesiod, Homer, and others. Their gods have human flaws, including jealousy. Perfect beings would not be troubled by anything, including the behavior of humans. Hence, the notion that the gods will reward or punish us is absurd. To be perfect is to be unperturbed. The concept of perfection, therefore, requires that the gods be indifferent to human behavior. Some have rejected belief in the Christian God for similar reasons. The idea of a perfect being creating the universe is self-contradictory. How can perfection be improved upon? To create is to indicate a lack, an imperfection. If that objection can be answered, another arises: if God is All-Good and All-Powerful, evil should not exist. Therefore, either God is All-Good but allows evil because God is not All-Powerful, or God is All-Powerful but allows evil because God is not All-Good. Such an argument clearly does not deny the existence of all gods.

Others have rejected the Christian God because they believe that the concept of worship, essential to most Christians, contradicts the concept of omnipotence (Rachels 1989). Still others reject a belief in the JCIG because they consider the scriptures used to support that belief to be unbelievable. Some theologians have tried to prove through reason alone that this God exists. Rejection of such proofs, however, is not atheism.

Some Christians consider Buddhists to be atheists, apparently for the same reason they consider Spinoza or Plato to be atheists: Anyone who rejects the Omnipotent and All-Good Providential Personal Creator rejects God. Yet, rejecting the JCIG is not to reject all gods. Nor is rejecting the JCIG the same as rejecting belief in an ultimate ground or principle of being and goodness, a being that explains both why there is something rather than nothing and why everything is as it is. Nor is rejection of the JCIG the same as rejecting belief in a realm of beings such as devas or spirits that are not limited by mortality and other human or animal frailties.

Finally, atheists do not deny that people have ‘mystical’ or ‘religious’ experiences, where one feels God’s presence or a sense of the oneness and significance of everything in the universe. Nor do atheists deny that many people experience God’s presence in their everyday lives. Atheists deny that the brain states that result in such feelings and experiences have supernatural causes.

How widespread is atheism? A worldwide survey in 2000 by the Gallup polling agency found that 8% do not think there is in any spirit, personal God, or life force. Another 17% are not sure. However, more than half the world’s population, and more than 90% of the world’s scientists, do not believe in a personal God, and hence would be considered atheists by many Christians.