Researched by the Ignaria Editorial Team · Updated 2026-05-20
The early Church's posture toward war shifted dramatically between the pre-Constantinian period — when most writers counseled nonparticipation in Roman military service — and the post-Constantinian era, when Augustine developed the first systematic Christian just war theory. Augustine drew on Cicero's criteria and baptized them within a theology of love: war can be just when it aims at restoring peace, is authorized by legitimate authority, and is waged with the intention of benefiting even the enemy. Aquinas systematized Augustine's criteria into the three requirements that became the medieval foundation: just cause, right intention, and legitimate authority.
Scriptural Foundations for Regulated Warfare
Old Testament law establishes that even divinely sanctioned conflict operates under constraints. Deuteronomy 20 requires an offer of peace before siege and distinguishes between combatants and non-combatants — evidence that divine authorization for war does not eliminate moral boundaries. Matthew Henry's description of the church as "militant" frames Christian life itself as a form of warfare requiring order and purpose, connecting regulated physical conflict to the church's ongoing spiritual struggle.
Early Christian Distinctions on Military Service
Augustine of Hippo articulated the foundational Christian distinction: the real evils of war are disordered passions — "love of violence, revengeful cruelty, fierce and implacable enmity" — not the act of fighting itself. Good men may wage war in obedience to lawful authority to punish these evils. Crucially, Augustine insisted that even in waging war, soldiers must "cherish the spirit of a peacemaker": the goal of conquest is to restore peace to the defeated, not to destroy or dominate.
Medieval Systematization of Just War Criteria
Thomas Aquinas systematized Augustine's insights into explicit criteria. Wars are "peaceful" in their fundamental orientation when they aim at securing peace, punishing wrongdoers, and promoting the good — not at aggrandizement or cruelty. Aquinas also separated the church's spiritual ministry from civil authority's role in using force, ruling that "warlike pursuits are altogether incompatible with the duties of a bishop and a cleric" — preserving the distinction between those who bear the sword and those who minister the Word.
Reformation-Era Applications and Qualifications
Reformation theologians applied just war criteria to contemporary political realities. Calvin grounded the legitimacy of war in "public vengeance" — whether it serves the execution of public justice rather than private interest. Zwingli drew a sharp contrast between mercenary service (self-interested and condemned) and fighting for truth, religion, justice, and native country, which he called a "sacred cause." Both reformers insisted that motivation distinguishes lawful war from mere violence.
What the primary sources show
"And if it will make no peace with thee, but will make war against thee, then thou shalt besiege it" — the requirement to offer peace before siege, with distinctions between combatants and non-combatants, establishes that even divinely authorized warfare operates under moral constraints rather than as unrestricted violence.
"The church being militant, those only are reputed the true members of it that have enlisted themselves soldiers of Jesus Christ; for our life, our Christian life, is a warfare." — Henry connects regulated warfare imagery to the spiritual life of believers, framing Christian existence as requiring the same order and purpose that governs physical conflict.
"The real evils in war are love of violence, revengeful cruelty, fierce and implacable enmity, wild resistance, and the lust of power, and such like; and it is generally to punish these things… that, in obedience to God or some lawful authority, good men undertake wars." — Augustine's decisive distinction: warfare itself is not the evil; disordered passion is.
"Therefore, even in waging war, cherish the spirit of a peacemaker, that, by conquering those whom you attack, you may lead them back to the advantages of peace." — Augustine insists the Christian warrior must not abandon the gospel commitment to peace; the conquered should be brought to peace's benefits, not perpetual enmity.
"In order for a war to be just, three things are necessary. First, the authority of the sovereign by whose command the war is to be waged. For it is not the business of a private individual to declare war, because he can seek for redress of his rights from the tribunal of his superior... And as the care of the common weal is committed to those who are in authority, it is their business to watch over the common weal of the city, kingdom or province subject to them." — Aquinas's first criterion for just war (right authority); he goes on to require a just cause and right intention, systematizing Augustine's framework into the medieval foundation for Christian just war theory.
"As it is sometimes necessary for kings and states to take up arms in order to execute public vengeance, the reason assigned furnishes us with the means of estimating how far the wars which are thus undertaken are lawful." — Calvin evaluates war's legitimacy by whether it serves public justice rather than private interest.
"Those who, for truth, religion, justice, and native country, venture their lives in war, are true men, and their cause is sacred." — Zwingli contrasts the sacred cause of those fighting for fundamental goods with the condemned self-interest of mercenary service.