Miles Christianus

[3] By the 5th century, the Church had started to develop doctrines that allowed for Christian participation in battle, though this was limited by a requirement that the fighting must be undertaken to convert infidels or spread the glory of Christ.

[4] The phrase miles Christi, derived from a letter from Paul the Apostle and much employed by Pope Gregory VII, also appeared in the Gesta Francorum in reference to the young Prince Tancred, Bohemond, Godfrey and Count Raymond of Toulouse,[5] each of whom were Christian leaders in the First Crusade.

The metaphor has its origins in early Christianity of the Roman Empire, and gave rise to the contrasting term paganus (hitherto used in the sense of "civilian," "one lacking discipline") for its opposite, i.e. one who was not a soldier of Christ.

This parallels the development of the understanding in medieval Christendom of the armed nobility as defenders of the faith, first emphasized by Gregory VII in the context of the Investiture controversy and later made even more explicit with the actual military expeditions of the crusades.

Depictions of the miles christianus with the emblematic Armour of God however remained very rare in the medieval period and only became prominent after the Protestant Reformation.