The term infidel simply means ‘one without faith’.
It is an old-time term which refers to somebody who goes against the tenets and/or beliefs of their regional religion.
This, of course, means that while somebody may be considered an infidel in their native region, they may or may not be considered an infidel elsewhere.
The term infidel is not synonymous with atheism.
Definition of Infidel
The Oxford English Dictionary describes the word infidel as:
1. One who does not believe in (what the speaker holds to be) the true religion; an ‘unbeliever’.
2. In specific applications: a. From a Christian point of view: An adherent of a religion opposed to Christianity; esp. a Muhammadan, a Saracen (the earliest sense in Eng.); also (more rarely), applied to a Jew, or a pagan. Now chiefly Hist.
2.b From a non-Christian (esp. Jewish or Muhammadan) point of view: Gentile, Giaour, etc.
3.a. disbeliever in religion or divine revelation generally; especially one in a Christian land who professedly rejects or denies the divine origin and authority of Christianity; a professed unbeliever. Usually a term of opprobrium.
b. Of persons: Unbelieving; adhering to a false religion; pagan, heathen, etc. (Cf. the n.)
Historically, the term infidel was generally used in a negative context, but that isn’t always the case.
Some people may use the term loosely, to describe people of alternative beliefs, but this has become less common in recent times.
Redefining the Term Infidel
During the period of Enlightenment, some people began to adopt the term infidel, displacing the notion of it’s negative context.
Some of them wore the label as a symbol of pride, and refused to hide from the connotations associated with it.
It was during this time that many philosophers and skeptics chose to forgo the religious institutions, instead placing their belief in the world around them.
Although not all members of this group were atheistic, most of them were.
The term fell out of favor in the modern era, and the term ‘secularism’ is becoming more popular, as it is a term which can be used to describe both believers and non-believers.