Throughout the course of history, human beings have searched for objective, constant, universal, and authoritative standards of justice that would not only serve to govern their individual and collective conduct, but also serve as standards that could be used to evaluate the actions of others within society. This quest was particularly compelling with respect to questions relating to justice in war: specifically, what just criteria should be use to determine whether to resort to the use of armed force as an instrument of policy in the resolution of conflicts and, if force is to be used, how might it be justly applied both during and after the termination of hostilities? Christian thinkers of the late Roman, medieval, and early modern periods believed that, through a synergistic blend of faith and reason, they could secure answers to these questions. The purpose of this chapter is to analyze these efforts. Hence, the first portion of the chapter will analyze the fundamental Christian beliefs that would, in turn, condition their quest to determine standards of just conduct. The second portion of the chapter will then explore the ways in which these thinkers built upon their core beliefs and concepts of justice to gradually formulate their individual criteria for “just war.”