Someone who does not believe in religion is often referred to as an "atheist." An atheist is a person who lacks belief in the existence of deities or gods. It's essential to note that atheism is not a unified belief system or religion; it is simply the absence of belief in any god or gods. Some atheists may actively assert that there are no deities (strong atheism), while others may simply lack belief without making any positive claims (weak atheism or agnostic atheism).