Atheists base their position on a more active logical analysis

Atheism entails, minimally, the absence of belief in the existence of any deities. It is contrasted with theism, the belief in a God or gods. Atheism is commonly defined as the positive belief that deities do not exist, or as the deliberate rejection of theism. However, others define atheism as the simple absence of belief in deities, thereby designating all agnostics, and people who have never heard of gods, such as newborn children, as atheists as well. In recent years, some atheists have adopted the terms strong and weak atheism to clarify whether they consider their stance one of positive belief (strong atheism) or the mere absence of belief (weak atheism).
Many self-described atheists share common skeptical concerns regarding supernatural claims, citing a lack of empirical evidence for the existence of deities. Other rationales for atheism range from the philosophical to the social to the historical. Although atheists tend toward secular philosophies such as humanism, rationalism, and naturalism, there is no one ideology or set of behaviors that all atheists adhere to.
In Western culture, atheists are frequently assumed to be irreligious or non-spiritual. However, some religious and spiritual beliefs, such as several forms of Buddhism, have been described by outside observers as conforming to the broader, negative definition of atheism due to their lack of any participating deities. Atheism is also sometimes equated with antitheism (opposition to theism) or antireligion (opposition to religion), despite many atheists not holding such views.
Philosophical and Logical Reasons

Many atheists will point out that in philosophy and science, the default position on any matter is a lack of belief. If reliable evidence or sound arguments are not presented in support of a belief, then the "burden of proof" remains upon believers, not nonbelievers, to justify their view. Consequently, many atheists assert that they are not theists simply because they remain unconvinced by theistic arguments and evidence. As such, many atheists have argued against the most famous proposed proofs of God's existence, including the ontological, cosmological, and teleological arguments.

Other atheists base their position on a more active logical analysis, and subsequent rejection, of theistic claims. The arguments against the existence of God aim at showing that the traditional Judeo-Christian conception of God either is inherently meaningless, is internally inconsistent, or contradicts known scientific or historical facts, and that therefore a god thus described does not exist.

The most common of these arguments is the problem of evil, which Christian apologist William Lane Craig has called "atheism's killer argument." The argument is that the presence of evil in the world disproves the existence of any god that is simultaneously benevolent and omnipotent, because any benevolent god would want to eliminate evil, and any omnipotent god would be able to do so. Theists commonly respond by invoking free will to justify evil (cf. argument from free will). However, this leaves unresolved the related argument from nonbelief, also known as the argument from divine hiddenness, which states that if an omnipotent God existed and wanted to be believed in by all, it would prove its existence to all because it would invariably be able to do so. Since there are unbelievers, either there is no omnipotent God or God does not want to be believed in.

Wisdom Of Life

Another such argument is theological noncognitivism, which holds that religious language, and specifically words like God, is not cognitively meaningful. This argument was popular in the early 20th century among logical positivists such as Rudolph Carnap and A.J. Ayer, who held that talk of deities is literally nonsense. Such arguments have since fallen into disfavor among philosophers, but continue to see use among ignostics, who view the question of whether deities exist as meaningless or unanswerable, and apatheists, who view it as entirely irrelevant. Similarly, the transcendental argument for the non-existence of God , a reversal of the more well-known theistic argument, argues that logic, science, and morality can only be justified by appealing to a non-theistic worldview.