Roland Bainton divided Christian perspectives on war into three categories - pacifist, just war, and crusade. James Turner Johnson ( Just war tradition and the restraint of war: A moral and historical inquiry ) does not think Bainton’s categories are helpful.

For starters, the crusaders considered their wars to be just wars and used just war arguments to defend their participation. On the other end of the spectrum, Johnson points out (as do some “pacifists”) that there is not a single kind of pacifism in the Christian tradition. He concludes, “Like the concept of just war, that of pacifism is no absolute. Rather, there are many forms, theoretical and historical, of both.”

He also thinks that Bainton’s categories obscure the “strong antiviolence sentiment that has motivated much of the historical development of just war thought.”

This bias against violence undergirded what Johnson calls the “original just war question”: Is it ever justifiable for Christians to participate in war? Just war theorists answered yes, but that means “just war theory permits Christians to participate in one particular form of violence under certain specified conditions ” and at the same time limits that participation. If pacifism involves the repudiation of all violence, he notes, some who are called “pacifists” are really closer to just warriors (xxv-xxvii).

One of the problems addressed by the just war tradition is the issue of certainty. Augustine pointed out that Israel could know that their conquest was just because God commanded it. But where there is no command from God, Christians “must be very careful in taking up the sword against others.” This caution was reinforced liturgically in the middle ages, when the church required “that, after wars, soldiers do penance for the sins they might have committed while in arms” (xxx).