Which belief system actively denies the existence of any deity?

Atheism is the belief system that actively denies the existence of any deity. It posits that there are no gods or divine beings, placing a strong emphasis on a secular worldview. Atheists often base their beliefs on scientific reasoning, evidence, and logic, rejecting the claims made by various religions regarding the existence of a deity.

This stance sets atheism apart from other belief systems like agnosticism, which maintains that the existence of a deity is unknown or unknowable; theism, which affirms the belief in one or more deities; and spirituality, which often embraces a broader, more personal identification with the concepts of the divine or a higher power without adhering to specific religious doctrines. Atheism's clear position on the non-existence of deity is what fundamentally defines it within the context of belief systems.