Knights of Christ may safely fight the battles of their Lord, fearing neither sin if they smite the enemy, nor danger at their own death; since to inflict death or to die for Christ is no sin, but rather, an abundant claim to glory. In the first case one gains for Christ, and in the second one gains Christ himself. The Lord freely accepts the death of the foe who has offended him, and yet more freely gives himself for the consolation of his fallen knight.
The knight of Christ, I say, may strike with confidence and die yet more confidently, for he serves Christ when he strikes, and serves himself when he falls. Neither does he bear the sword in vain, for he is God’s minister, for the punishment of evildoers and for the praise of the good. If he kills an evildoer, he is not a mankiller, but, if I may so put it, a killer of evil. He is evidently the avenger of Christ towards evildoers, and he is rightly considered a defender of Christians. Should he be killed himself, we know that he has not perished, but has come safely into port. When he inflicts death, it is to Christ’s profit, and when he suffers death, it is for his own gain. The Christian glories in the death of the pagan because Christ is glorified; while the death of the Christian gives occasion for the King to show his liberality in the rewarding of his knight. In the one case the just shall rejoice when he sees justice done, and in the other man shall say, truly there is a reward for the just; truly it is God who judges the earth.
I do not mean to say that the pagans are to be slaughtered when there is any other way to prevent them from harassing and persecuting the faithful, but only that it now seems better to destroy them than that the rod of sinners be lifted over the lot of the just, and the righteous perhaps put forth their hands unto iniquity.
What then? If it is never permissible for a Christian to strike with the sword, why did the Savior’s precursor bid the soldiers to be content with their pay, and not rather forbid them to follow this calling? But if it is permitted to all those so destined by God, as is indeed the case, provided they have not embraced a higher calling, to whom, I ask, may it be allowed more rightly than to those whose hands and hearts hold for us Sion, the city of our strength?
Thus, when the transgressors of divine law have been expelled, the righteous nation that keeps the truth may enter in security. Certainly, it is proper that the nations who love war should be scattered, that those who trouble us should be cut off, and that all the workers of iniquity should be dispersed from the city of the Lord. They busy themselves to carry away the incalculable riches placed in Jerusalem by the Christian peoples, to profane the holy things and to possess the sanctuary of God as their heritage. Let both swords of the faithful fall upon the necks of the foe, in order to destroy every high thing exalting itself against the knowledge of God, which is the Christian faith. – from In Praise of the New Knighthood (c. 1136)