An Atheist is someone who lacks a belief in a god or gods. The critical characteristic is belief. Anyone who does not have an affirmative belief in at least one god is an Atheist. Beliefs are a result of knowledge, experience and feelings. While beliefs can change, they are not the result of a willful process. One either has a belief or doesn't. One cannot decide whether or not to have a belief. Faith is a belief maintained without supporting evidence or in the face of controverting evidence. Atheists do not have faith in god because they do not believe that any god exists. Hard Atheism, also known as Anti-theism, is the affirmative belief that no gods exists.
There are many gods that people have worshiped. Everyone is an Atheist with regards to gods in which they don’t believe, for example, Zeus, Thor, and Huitzilopotchli are worshiped by very few people. People are not born believing in god. Typically, people are born into a family that worships a particular god or gods and taught the culture, history and texts associated with a religion. One may or may not be convinced to believe and worship in the religion. Atheists may have been born into Atheist family, never convinced to believe in their family’s religion or converted from a religion to Atheism. The transition from religious to Atheist may take many years and go through many stages of belief, for example, from not believing in the hierarchy of a particular religion, to not believing in the doctrine of a particular religion, to not believing in a particular form of god, to not believing in any form of god. Atheists may be assumed to be immoral or evil in a religious community. Many people who are Atheists have not told their family, friends and co-workers that they are Atheist because of the social repercussions, from ostracism to death, of being an Atheist in a believing community.
An Agnostic lacks knowledge. An Agnostic Atheist does not believe in god because they do not know that god exists. At least one god either exists or does not exist. Whether or not one wants a god to exist does not affect whether or not a god actually exists. An agnostic atheist has not been convinced that god exists because of a lack of evidence that any god exists.
In a debate, the theistic claim that there is a god carries the philosophical burden of proof because it makes an affirmative claim that god exists. Many theists believe that there is evidence for god based on religious texts, subjective experience or the apparent design of the universe. However, texts are written and interpreted by humans. Fictional religious figures may be mistaken for historical figures. The appearance of known locations or historical figures in a religious text do not guarantee that the text is true in its supernatural claims. The feelings and behaviors described by believers as evidence of god can be felt and experienced by non-believers in ecstatic states, including talking in tongues, or as a result of mental or physical impairment. Also, the apparent design of the universe and life is explainable by the alternative hypotheses of the Big Bang Theory and Evolutionary Theory. The Big Bang Theory explains how the universe developed from the explosion of a singularity. How the physical constants of the universe were determined is not known. Some posit that they may have been determined by chance given a multiverse of infinite universes, but this is currently speculation. Evolutionary Theory explains how life adapts to its environment over generations. The mechanism of how life arose, Abiogenesis, is not fully understood, but is under investigation.
In science, a theory is not simply an idea or speculation. A theory is an explanation of a phenomenon in the natural world confirmed based on repeated observation and experimentation subjected to peer review. Peer review is when other experts in a subject review evidence to determine if the appropriate methods have been used and analysis have been done to provide credible evidence. If a phenomenon is not well known or understood but someone has an idea of how it may be explained, then a hypothesis may be put forward, but it will not be accepted as theory without rigorous testing. If there is no known explanation for a phenomenon, then one can choose to try explore possible hypotheses or say, “I don’t know.” Just because something is not known does not mean that god can be assumed to be the explanation. There may be an explanation in the natural world that has not yet been discovered or understood. Existing theories can be overturned or modified based on new evidence, but it is rare that an entire theoretical framework is disrupted. More often, additional hypotheses are put forward and tested at the limits of a theory, modifying or enhancing specific explanations within a larger theoretical framework.
A Gnostic Atheist claims to know that gods do not exist. However, the universe is so large and we have such a limited view of it and what may be beyond it that it is impossible to truly know that god does not exist. If there is a being with access to the fifth dimension, that being would appear to have the qualities of omniscience and omnipotence and possibly the powers often ascribed to gods. It may have the power to create in our four dimensional world the way that we create drawings and sculptures in two and three dimensions. However, even if such a being exists, it is unlikely to be alone and it must have originated somehow, which would also be a study of the natural world, just an aspect of it that we don’t normally experience, like quantum mechanics at the atomic level. There is no reason to worship such a being other than out of fear and ignorance. If such a being exists, then we should try to communicate with it and understand it. However, there is no evidence that any such being exists.