I. The principle of the Mosaic law and it is a principle of no little importance in its own place is that there should be as far as possible a just proportion between the offence and the punishment; that the penalty to be inflicted should neither be too light nor too severe, but that the one should be a fair equivalent for the other. While granting to the full the exceeding wisdom of the Mosaic law on this head, I must add that even in its judicial aspect it by no means comes up to the requirements of the Gospel. There is nothing indeed more beautiful than justice and more Divine. But Christian men, Christian society, Christian legislators, have other duties even to the criminal population besides punishing their offences. It may be necessary, it is necessary, to inflict punishment on the wrongdoer; but it is equally necessary to put away all wrath and revenge, and go to him in the spirit of brotherly love, and heap also coals of fire on his head, to turn him, if possible, to better thoughts and better ways.
II. For the right understanding of what our Lord says here it must be remembered that, while this law properly belonged to the judicial procedure of Israel, it was often applied by the people as a rule of private conduct. Our Lord is here dealing in general with the principle of private revenge, which He is anxious to destroy, because it is most fatal at once to the spiritual and social life of men. But, as usual, He goes for this purpose down into principles of moral duty, which lie far deeper than the precise question on hand; because His object is not merely to prevent a certain evil from being done, but to implant another spirit altogether in our hearts. Therefore He tells them that they are not only not to avenge themselves, but that they are not even to resist evil, but rather to overcome evil with good. Evil is never overcome with evil, but only with good. Your fire will not put out your neighbour's; rather they will combine and make a double conflagration, his wrong and your wrath together vexing the world.
W. C. Smith, The Sermon on the Mount,p. 131.
References: Matthew 5:38-42. J. Oswald Dykes, The Laws of the Kingdom,p. 87; Ibid., The Manifesto of the King,p. 287. Matthew 5:39. H. Melvill, Penny Pulpit,No. 1,552.Matthew 5:39-41. W. Gresley, Practical Sermons,p. 257. Matthew 5:39-42. E. J. Hardy, Faint, yet Pursuing,p. 258.