Most of what you read in the New Testament has a counterpart in the Old Testament. For example, the “love your enemies” principle is found throughout the entire Bible
If thine enemy be hungry, give him bread to eat; and if he be thirsty, give him water to drink: For thou shalt heap coals of fire upon his head, and the LORD shall reward thee (Proverbs 25: 21, 22 KJV).
If you come across your enemy’s ox or donkey wandering off, be sure to take it back to him. If you see the donkey of someone who hates you fallen down under its load, do not leave it there; be sure you help him with it (Exodus 23: 4, 5 NIV).
And the king of Israel said unto Elisha,when he saw them [the enemies of Israel], My father, shall I smite them? Shall I smite them? And he [Elisha] answered, Thou shalt not smite them: wouldest thou smite those whom thou hast taken captive with thy sword and with thy bow? set bread and water before them, that they may eat and drink, and go to their master (II Kings 6: 21, 22 KJV)
Dearly beloved, avenge not yourselves, but rather give place unto wrath: for it is written, Vengeance is mine; I will repay, saith the Lord. Therefore if thine enemy hunger, feed him; if he thirst, give him drink: for in so doing thou shalt heap coals of fire on his head. Be not overcome of evil, but overcome evil with good (Romans: 12: 19-21 KJV).
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; (Matthew 5: 43, 44 KJV).