When Frederick the II unilaterally arrogated papal authority, he took on the mantle to "destroy convert, and subjugate all barbarian nations." A power in papal doctrine reserved for the pope.
During the Age of Discovery, the Papal Bulls such as Romanus Pontifex and more importantly inter caetera (1493), implicitly removed dominium from infidels and granted them to the Spanish Empire and Portugal with the charter of guaranteeing the safety of missionaries.
Subsequent English and French rejections of the bull refuted the Popes authority to exclude other Christian princes.
Later during the Victorian era, testimony of either self declared, or those accused of being Infidels or Atheists, was not accepted in a court of law because it was felt that they had no moral imperative to not lie under oath because they did not believe in God, or Heaven and Hell.
In the Quran the term kafir is first applied to the unbelieving Meccans, and their attempts to refute and revile Muhammad.
This is because marriage is a sacrament of the Catholic Church, which infidels are deemed incapable of receiving.
Some philosophers such as Thomas Paine, David Hume, George Holyoake, Charles Bradlaugh, Voltaire and Rousseau earned the label of infidel or freethinkers, both personally and for their respective traditions of thought because of their attacks on religion and opposition to the Church.
The word originally denoted a person of a religion other than one's own, specifically a Christian to a Muslim, a Muslim to a Christian, or a gentile to a Jew.
Later meanings in the 15th century include "unbelieving", "a non-Christian" and "one who does not believe in religion" (1527).
Infidel (literally "unfaithful") is a term used in certain religions for those accused of unbelief in the central tenets of their own religion, for members of another religion, or for the irreligious.