Infidel (literally "unfaithful") is a pejorative term used in certain religions for those who do not believe the central tenets of one's own religion, are members of another religion, or are not religious.
Infidel is an ecclesiastical term in Christianity around which the Church developed a body of theology that deals with the concept of infidelity, which makes a clear differentiation between those who were baptized and followed the teachings of the Church versus those who are outside the faith. The term infidel was used by Christians to describe those perceived as the enemies of Christianity.
After the ancient world the concept of otherness, an exclusionary notion of the outside by societies with more or less coherent cultural boundaries, became associated with the development of the monotheistic and prophetic religions of Judaism, Christianity and Islam.
In modern era literature, the term infidel includes in its scope atheists, polytheists, animists, heathen and pagan. Infidel as a concept is sometimes contrasted with the concept of religious pluralism.