From Latin infidēlis (unfaithful) = in- (not) + fidēlis (faithful), from fidēs (faith), PIE *bʰeydʰ- (to trust). A Crusade-era label that was always reciprocal — Christians called Muslims infidels while Muslims called Christians kāfir. The Enlightenment repurposed it for atheists. The same root of trust that gives us faith, fidelity, and confidence also produces this word for its opposite.
A person who does not believe in a particular religion, especially one regarded as the dominant faith; historically applied by Christians to Muslims and vice versa, and later to atheists and freethinkers.
From Latin 'infidēlis' (unfaithful, unbelieving), composed of 'in-' (not, negating prefix, from PIE *ne-, universal negator) + 'fidēlis' (faithful, trustworthy), from 'fidēs' (faith, trust, loyalty), from PIE *bʰeydʰ- (to trust, to have confidence in). The root *bʰeydʰ- generated a remarkably coherent family: Latin 'fīdus' (faithful), 'foedus' (treaty, covenant), 'fidēs' (faith — source of 'fidelity', 'fiancé', 'affidavit'), Greek 'peíthein' (to persuade), 'peistós' (persuasive), and Gothic 'beidan' (to wait, trust). 'Infidel' entered English around 1460 via Old