While polytheism dominated much of ancient Greek thought and theism dominated medieval Christian view, atheism has had its day in the modern world. Of course not all who lack faith in a divine being wish to be called "atheist." Some prefer the positive ascription of "Humanist". Others are perhaps best described as "materialists." But all are non-theists, and most are antitheistic. Some prefer the more neutral term a-theists.

In distinction from a theist, who believes God exists beyond and in the world, and a pantheist, who believes God is the world, an atheist believes there is no God either beyond or in the world. There is only a universe or cosmos and nothing more.

Since atheists share much in common with agnostics and skeptics, they are often confused with them (see Russell, "What Is an Agnostic?"). Technically, a skeptic says "I doubt that God exists" and an agnostic declares "I don’t know (or can’t know) whether God exists." But an atheist claims to know (or at least believe) that God does not exist. However, since atheists are all nontheists and since most atheists share with skeptics an antitheistic stand, many of their arguments are the same. It is in this sense that modern atheism rests heavily upon the skepticism of David Hume and the agnosticism of Immanuel Kant.

Broadly speaking, there are differing kinds of atheism. Traditional (metaphysical) atheism holds that there never was, is, or will be a God. The many with this view include Ludwig Feuerbach, Karl Marx, Jean-Paul Sartre, and Antony Flew. Mythological atheists, such as Friedrich Nietzsche, believe the God-myth was never a Being, but was once a live model by which people lived. This myth has been killed by the advancement of man’s understanding and culture. There was a short-lived form of dialectical atheism held by Thomas Altizer which proposed that the once-alive, transcendent God actually died in the incarnation and crucifixion of Christ, and this death was subsequently realized in modern times. Semantical atheists claim that God-talk is dead. This view was held by Paul Van Buren and others influenced by the logical positivists who had seriously challenged the meaningfulness of language about God. Of course, those who hold this latter view need not be actual atheists at all. They can admit to the existence of God and yet believe that it is not possible to talk about him in meaningful terms. This view has been called "acognosticism," since it denies that we can speak of God in cognitive or meaningful terms. Conceptual atheism believes that there is a God, but he is hidden from view, obscured by our conceptual constructions. Finally, practical atheists confess that God exists but believe that we should live as if he did not. The point is that we should not use God as a crutch for our failure to act in a spiritual and responsible way (some of Dietrich Bonhoffer’s writings can be interpreted in this category).

There are other ways to designate the various kinds of atheists. One way would be by the philosophy by which they express their atheism. In this way one could speak of existential atheists (Sartre), Marxist atheists (Marx), psychological atheists (Sigmund Freud), capitalistic atheists (Ayn Rand), and behavioristic atheists (B. F. Skinner).

For apologetics purposes the most applicable way to consider atheism is in a metaphysical sense. Atheists are those who give reasons for believing that no God exists in or beyond the world. Thus we are speaking about philosophical atheism as opposed to practical atheists who simply live as though there were no God.

The arguments for atheism are largely negative, although some can be cast in positive terms. Negative arguments fall into two categories: (1) arguments against proofs for God’s existence, and (2) arguments against God’s existence. On the first set of arguments most atheists draw heavily on the skepticism of Hume and the agnosticism of Kant.

Atheists offer what they consider to be good and sufficient reasons for believing no God exists. Four such arguments are often used by atheists: (1) the fact of evil; (2) the apparent purposelessness of life; (3) random occurrence in the universe; and (4) the First Law of Thermodynamics—that "energy can neither be created or destroyed"— as evidence that the universe is eternal and, hence, needs no Creator.

The Existence of Evil. A detailed response to the problem of evil is given elsewhere (see The Problem of Evil, Theological Dictionary, October & November, 1999), so it will be treated here only in general terms. The atheist’s reasoning is circular. Former atheist C. S. Lewis argued that, in order to know there is injustice in the world one has to have a standard of justice. So, to effectively eliminate God via evil one has to posit an ultimate moral standard by which to pronounce God evil (Mere Christianity). But for theists God is the ultimate moral standard, since there cannot be an ultimate moral law without an Ultimate Moral Law Giver.

Atheists argue that an absolutely good God must have a good purpose for everything, but there is no good purpose for much of the evil in the world. Hence, there cannot be an absolutely perfect God.

Theists point out that just because we do not know the purpose for evil occurrences does not mean that there is no good purpose. This argument does not necessarily disprove God; it only proves our ignorance of God’s plan. Along the same reasoning, just because we do not see a purpose for all evil now, does not follow that we never will. The atheist is premature in his judgment. According to theism, a day of justice is coming. If there is a God, he must have a good purpose for evil, even if we do not know it. For a theistic God is omniscient and knows everything. He is omnibenevolent and has a good reason for everything. So, by his very nature he must have a good reason for evil.

Purposelessness. In assuming that life is without purpose, the atheist is again both a presumptuous and premature judge. How does one know there is no ultimate purpose in the universe? Simply because the atheist knows no real purpose for life does not mean God does not have one. Most people have known times that made no sense for the moment but eventually seemed to have great purpose.

The Random Universe. Apparent randomness in the universe does not disprove God. Some randomness is only apparent, not real. When DNA was first discovered it was believed that it split randomly. Now the entire scientific world knows the incredible design involved in the splitting of the double helix molecule known as DNA. Even actual randomness has an intelligent purpose. Molecules of carbon dioxide are exhaled randomly with the oxygen (and nythogine in the air), but for a good purpose. If they did not, we would inhale the same poisonous gases we have exhaled. And some of what seems to be waste may be the product of a purposeful process. Horse manure makes good fertilizer. According to the atheist’s time scale the universe has been absorbing and neutralizing very well all its "waste." So far as we know, little so-called waste is really wasted. Even if there is some, it may be a necessary byproduct of a good process in a finite world like ours, just like sawdust results from logging.

The Eternality of Matter (Energy). Atheists often misstate the scientific first law of thermodynamics. It should not be rendered: "Energy can neither be created nor destroyed." Science as science should not be engaged in "can" or "cannot" statements. Operation science deals with what is or is not, based on observation. And observation simply tells us, according to the first law, that "The amount of actual energy in the universe remains constant." That is, while the amount of usable energy is decreasing, the amount of actual energy is remaining constant in the universe. The first law says absolutely nothing about the origin or destruction of energy. It is merely an observation about the continuing presence of energy in the cosmos.

Unlike the second law of thermodynamics, which tells us the universe is running out of usable energy and, hence, must have had a beginning, the first law makes no statement about whether energy is eternal. Therefore, it cannot be used to eliminate a Creator of the cosmos.