Laudator Temporis Acti

In the Sermon on the Mount (Matthew 5.43-46) Jesus said:

Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy. But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you; That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust. For if ye love them which love you, what reward have ye? do not even the publicans the same?

Most ancient Greeks before the time of Christ would have regarded this notion as utterly preposterous. Among them it was a commonplace of morality, indeed almost the very essence of justice, that one should benefit one's friends and harm one's enemies. Denys L. Page, in his commentary on Euripides' Medea 809-810, quotes Lessing:

Moral excellence in ancient Greece consisted no less in unremitting hatred of your foes than in unalterable love toward your friends.

Here is a selection of passages from Greek literature on the topic:

• Hesiod, Works and Days, 353-354 (tr. Hugh G. Evelyn White): Be friends with the friendly, and visit him who visits you. Give to one who gives, but do not give to one who does not give.
  
• Theognis 869-872 (tr. J.M. Edmonds): May the great wide brazen sky fall upon me -- that dread of earthborn men -- if I aid not such as love me, and become not a pain and great grief unto such as hate.
  
• Archilochus, fragment 23 West, lines 14-15: I know how to love the one who loves me and hate the enemy.
  
• Archilochus, fragment 126 West: One important thing I know, how to repay with terrible evils the one who mistreats me.
  
• Solon, fragment 13 West (a prayer to the Muses, lines 5-6, tr. J.M. Edmonds): Make me, I pray you, sweet to my friends and sour to my foes, to these a man reverend to behold, to those a man terrible.
  
• Pindar, Pythian Odes 2.83-85 (tr. William H. Race): Let me befriend a friend, but against an enemy, I shall, as his enemy, run him down as a wolf does, stalking now here, now there, on twisting paths.
  
• Aeschylus, Libation Bearers 122-123 (tr. Herbert Weir Smyth): ELECTRA. And is this a righteous thing for me to ask of Heaven? CHORUS. Righteous? How not? To requite an enemy evil for evil!
  
• Sophocles, Antigone 641-644 (tr. Hugh Lloyd-Jones): This is why men pray that they may beget and keep in their houses obedient offspring, so that they may requite the enemy with evil and honour the friend as they honour their father.
  
• Euripides, Medea 807-810 (tr. David Kovacs): Let no one think me weak, contemptible, untroublesome. No, quite the opposite, hurtful to foes, to friends kindly. Such persons live a life of greatest glory.
  
• Euripides, Heracles 585-586 (tr. David Kovacs): It is in your nature, my son, to be loving to your friends and to hate your enemies.
  
• Euripides, Ion 1046-1047 (tr. David Kovacs): But when a man wants to harm an enemy, no law stands in the way.
  
• Lysias 9.20 (tr. W.R.M. Lamb): I considered it ordained that one should harm one's enemies and serve one's friends.
  
• Xenophon, Memorabilia 2.6.35 (tr. E.C. Marchant): A man's virtue consists in outdoing his friends in kindness and his enemies in mischief.
  
• Xenophon, Anabasis 1.9.11 (tr. Carelton L. Brownson): It was manifest also that whenever a man conferred any benefit upon Cyrus or did him any harm, he always strove to outdo him; in fact, some people used to report it as a prayer of his that he might live long enough to outdo both those who benefited and those who injured him, returning like for like.
  
• Xenophon, Hiero 2.2 (tr. E.C. Marchant): You are rich in power to harm enemies and reward friends.
  
• Plato, Republic 1.7.332d (tr. B. Jowett): Justice is the art which gives good to friends and evil to enemies.

Polybius 1.14.4-5 (tr. W. R. Paton) allows an exception to the rule only in the person of the historian who is bound to be impartial:

In other relations of life we should not perhaps exclude all such favouritism; for a good man should love his friends and his country, he should share the hatreds and attachments of his friends; but he who assumes the character of a historian must ignore everything of the sort, and often, if their actions demand this, speak good of his enemies and honour them with the highest praises while criticizing and even reproaching roundly his closest friends, should the errors of their conduct impose this duty on him.