Matthew 5:43-48 Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy. 44 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; 45 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. 46 For if ye love them which love you, what reward have ye? do not even the publicans the same? 47 And if ye salute your brethren only, what do ye more than others? do not even the publicans so? 48 Be ye therefore perfect, even as your Father which is in heaven is perfect.
Under the Old Covenant made with Israel at Sinai a distinction was made between Israelites and other nations.
They were to love their neighbours
Leviticus 19:18 Thou shalt not avenge, nor bear any grudge against the children of thy people, but thou shalt love thy neighbour as thyself: I am the LORD.
They were to hate (love less) other nations.
The Canaanites, Ammonites and Moabites were to be counted as enemies.
Deuteronomy 20:17 But thou shalt utterly destroy them; namely, the Hittites, and the Amorites, the Canaanites, and the Perizzites, the Hivites, and the Jebusites; as the LORD thy God hath commanded thee:
Deuteronomy 23:3-6 An Ammonite or Moabite shall not enter into the congregation of the LORD; even to their tenth generation shall they not enter into the congregation of the LORD for ever: 4 Because they met you not with bread and with water in the way, when ye came forth out of Egypt; and because they hired against thee Balaam the son of Beor of Pethor of Mesopotamia, to curse thee. 5 Nevertheless the LORD thy God would not hearken unto Balaam; but the LORD thy God turned the curse into a blessing unto thee, because the LORD thy God loved thee. 6 Thou shalt not seek their peace nor their prosperity all thy days for ever.
Israelites consequently tended to treat all non-Jews as enemies.
The New Covenant in Christ was to be opened up to all peoples, and believers were now to treat friends, neighbours and enemies alike. Those who are baptised into Christ are to love and do good to all, in the same way that God does.
Matthew 10:16 Behold, I send you forth as sheep in the midst of wolves: be ye therefore wise as serpents, and harmless as doves.
Galatians 6:10 As we have therefore opportunity, let us do good unto all men, especially unto them who are of the household of faith.
This commandment means that those in Christ cannot become involved in armed conflict or civil disobedience. If we suffer wrong at the hands of others, we must leave retribution to God.
Matthew 5:39 But I say unto you, That ye resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also.
Luke 6:29 And unto him that smiteth thee on the one cheek offer also the other; and him that taketh away thy cloke forbid not to take thy coat also.
Romans 12:19 Dearly beloved, avenge not yourselves, but rather give place unto wrath: for it is written, Vengeance is mine; I will repay, saith the Lord.
This commandment amongst Christians tends to sort the wheat from the chaff.
I hope you find this helpful.