Augustine of Hippo — as excerpted in the Catena Aurea on Matthew 5:38-42

“(Serm. in Mont. i. 19.) For the righteousness of the Pharisees is a less righteousness, not to transgress the measure of equal retribution; and this is the beginning of peace; but perfect peace is to refuse all such retribution. Between that first manner then, which was not according to the Law, to wit, that a greater evil should be returned for a less, and this which the Lord enjoins to make His disciples perfect, to wit, that no evil should be returned for evil, a middle place is held by this, that an equal evil should be returned, which was thus the passage from extremest discord to extremest peace. Whoso then first does evil to another departs furthest from righteousness; and who does not first do any wrong, but when wronged repays with a heavier wrong, has departed somewhat from extreme injustice; he who repays only what he has received, gives up yet something more, for it were but strict right that he who is the first aggressor should receive a greater hurt than he inflicted. This righteousness thus partly begun, He perfects, who is come to fulfil the Law. The two steps that intervene He leaves to be understood; for there is who does not repay so much, but less; and there is yet above him, he who repays not at all; yet this seems too little to the Lord, if you be not also ready to suffer wrong. Therefore He says not, Render not evil for evil, but, Resist not against evil, not only repay not what is offered to you, but do not resist that it should not be done to you. For thus accordingly He explains that saying, If any man smite thee on thy right cheek, offer to him the left also. Which as being a high part of mercy, is known to those who serve such as they love much; from whom, being morose, or insane, they endure many things, and if it be for their health they offer themselves to endure more. The Lord then, the Physician of souls, teaches His disciples to endure with patience the sicknesses of those for whose spiritual health they should provide. For all wickedness comes of a sickness of the mind; nothing is more innocent than he who is sound and of perfect health in virtue.”