Writers who wrestled with the moral dimensions of worldly military service recognized the inner warfare waged in the spirit as a fundamental part of the good christian life. If engaging in spiritual combat kept one on the path to righteousness, winning a decisive victory in what Saint Augustine (d.430) called ‘the narrow theater of the heart’ marked one as a saint. Christ himself had taught men to do battle with the forces of evil, and those who wished to follow in his footsteps could find no better way than by devoting their lives to service in the militia Christi. But while the concept of spiritual warfare continued to fascinate christian thinkers throughout the Middle ages, its associations changed dramatically over time. This chapter delineates several key moments between the third and twelfth centuries when this idea was contested and reappropriated. For early christian writers the soldier of christ was the martyr, who fought beasts and gladiators in the arenas of the Roman Empire and won the prize of salvation through physical annihilation. In the fourth and fifth centuries the discourse of spiritual warfare was appropriated by promoters of the emerging monastic ideal, for whom the christian soldier was the ascetic who warred against demons in the desert. Martial spirituality powerfully shaped the development of the coenobitic life, whose apologists were as confident as their predecessors in their identification of the real milites Christi: these were the members of the monastic militia, drawn up against vices and demonic forces in an unbreakable battle line.