Definitions of atheism also vary in the degree of consideration a person must put to the idea of gods to be considered an atheist. Atheism has sometimes been defined to include the simple absence of belief that any deities exist. This broad definition would include newborns and other people who have not been exposed to theistic ideas. As far back as 1772, Baron d'Holbach said that "All children are born Atheists; they have no idea of God."  Similarly, George H. Smith (1979) suggested that: "The man who is unacquainted with theism is an atheist because he does not believe in a god. This category would also include the child with the conceptual capacity to grasp the issues involved, but who is still unaware of those issues. The fact that this child does not believe in god qualifies him as an atheist."  Smith coined the term implicit atheism to refer to "the absence of theistic belief without a conscious rejection of it" and explicit atheism to refer to the more common definition of conscious disbelief.

In Western civilization, the view that children are born atheist is relatively recent. Before the 18th century, the existence of God was so universally accepted in the western world that even the possibility of true atheism was questioned. This is called theistic innatism—the notion that all people believe in God from birth; within this view was the connotation that atheists are simply in denial. There is a position claiming that atheists are quick to believe in God in times of crisis, that atheists make deathbed conversions, or that "there are no atheists in foxholes." Some proponents of this view claim that the anthropological benefit of religion is that religious faith enables humans to endure hardships better, functioning as an "opium of the people". Some atheists emphasize the fact that there have been examples to the contrary, among them examples of literal "atheists in foxholes."

Positive vs. negative

Philosophers such as Antony Flew, and Michael Martin, have contrasted positive (strong/hard) atheism with negative (weak/soft) atheism. Positive atheism is the explicit affirmation that gods do not exist. Negative atheism includes all other forms of non-theism. According to this categorization, anyone who is not a theist is either a negative or a positive atheist. The terms weak and strong are relatively recent, while the terms negative and positive atheism are of older origin, having been used (in slightly different ways) in the philosophical literature and in Catholic apologetics since at least 1813. Under this demarcation of atheism, most agnostics qualify as negative atheists.

While Martin, for example, asserts that agnosticism entails negative atheism, most agnostics see their view as distinct from atheism, which they may consider no more justified than theism or requiring an equal conviction. The supposed unattainability of knowledge for or against the existence of gods is sometimes seen as indication that atheism requires a leap of faith. Common atheist responses to this argument include that unproven religious propositions deserve as much disbelief as all other unproven propositions, and that the unprovability of a god's existence does not imply equal probability of either possibility. Scottish philosopher J. J. C. Smart even argues that "sometimes a person who is really an atheist may describe herself, even passionately, as an agnostic because of unreasonable generalized philosophical skepticism which would preclude us from saying that we know anything whatever, except perhaps the truths of mathematics and formal logic." Consequently, some atheist authors such as Richard Dawkins prefer distinguishing theist, agnostic and atheist positions by the probability that each assigns to the statement "God exists".

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