Question Restatement: How should Jesus' command to "Not Resist, [Oppose], Evil" be interpreted--and to what extant?
(1.) Answer: How NOT to Oppose Evil
(2.) How TO Oppose Evil
(3.) What about Defending Others?
(4.) What about Self Defense, and other Contexts?
(5.) What if the Authorities Fail?
1. Answer: How NOT to Oppose Evil
Answer: Because of the text's context, the commandment certainly cannot be interpreted as, "Do not Oppose Evil", but rather it should be interpreted as "How NOT to Oppose Evil--by not Retaliating against the Person".
Context: This commandment sits within a series of Commandments affirming that Christians are--TO--Oppose and Overcome Evil, (Chapters 5-7), and more specifically, within the context of being "persecuted for the sake of righteousness/justice", and showing mercy, love, and forgiveness towards enemies..
Matt. 5:10-12, NASB - “Blessed are those who have been persecuted for the sake of righteousness
NOTE: In that culture, "Righteous" literally means "the one who acts justly", morally. And, "a Just/Righteous person" is recognized--even apart from the law. See Gen. 15:6, and Gal. 3:6.
Jesus, the Disciples, and the Early Church applied Jesus' command to the extreme--regardless of the severity of the evil done to them.
Jesus even rebuked Peter in the garden for using his sword--(even though Jesus told him to bring it). Stephen, and Paul demonstrated their interpretation of this commandment, and laid down their lives.
2. How TO Oppose Evil
Following these verses is an example of How TO Oppose Evil through mercy, and gentleness: that is:
Matt. 5:44, NASB - But I say to you, love your enemies, bless those who curse you, do good to those who hate you, and pray for those who spitefully use you and persecute you.
Loving Your Enemy, IS a form of Retaliation:
Prov. 25:21-21, NASB - If your enemy is hungry, give him food to eat; And if he is thirsty, give him water to drink; 22 For you will heap burning coals on his head, And the Lord will reward you.
To Oppose Evil--with Gentleness and Mercy--is Consistent in the New Testament:
Eph. 6:10-12, NASB - Finally, be strong in the Lord and in the strength of His might. 11 Put on the full armor of God, so that you will be able to stand firm against the schemes of the devil. 12 For our struggle is not against flesh and blood, but against the rulers, against the powers, against the world forces of this darkness, against the spiritual forces of wickedness in the heavenly places.
James 4:7, NASB - Submit therefore to God. Resist the devil and he will flee from you.
1 Peter 3:13-15, NASB - 13 Who is there to harm you if you prove zealous for what is good? 14 But even if you should suffer for the sake of righteousness, you are blessed. And do not fear their intimidation, and do not be troubled, 15 but sanctify Christ as Lord in your hearts, always being ready to make a defense to everyone who asks you to give an account for the hope that is in you, yet with gentleness and reverence;
3. What about Defending Others?
Jesus' commandment was certainly not commanding anyone to sit around and watch someone be abused--rather Jesus rebuked people for not providing justice, (Matt. 23:23), and then told them they were going to go to hell, (v. 33).
Matt 23:23, NASB - “Woe to you, scribes and Pharisees, hypocrites! For you tithe mint and dill and cummin, and have neglected the weightier provisions of the law: justice and mercy and faithfulness; but these are the things you should have done without neglecting the others.
... 33 You serpents, you brood of vipers, how will you escape the sentence of hell?
NOTE: This is often cited as a defense by Christians in the military, when challenged by other Christians who cite this command.
4. What about Self Defense, and other Contexts?
This commandment certainly does not preclude one from going to the authorities, especially to present a case about persecution for doing good because of an inward hope, (1 Peter 3:15, above).
Perhaps these commands only apply to actions within the context of "for the sake of righteousness / justice". Perhaps this commandment only applies to: "the mission trip to a third world country", or; the "Mission trip across the street", or; "the mission trip upstairs to their spouse".
Or perhaps, this context should apply to every act, which should be for the sake of righteousness / justice.
There are too many scenarios to consider, to show which exceptions should apply, so, the commandment must be addressed "Generally", at first :
Generally, Jesus affirmed that some instructions/commands were too difficult for some, because of the process of spiritual growth, a matter of faith, (like about not having sex, in Matt. 19:12).
The New Testament certainly affirms that in the end, obedience, (such as: keeping kosher, turning the "Other Cheek"), is absolutely a matter of personal faith, conviction, and trust :
That is, even if Jesus's commandment is to be interpreted--without exception, the person trying to obey this, outside of trusting/faith, would actually be sinning.
Rom. 14:23, NASB - 23 But he who doubts is condemned if he eats, because his eating is not from faith; and whatever is not from faith is sin.
5. What if the Authorities Fail?
What if the just person is persecuted, though acting justly/righteously, and appeals to family, friends, their community, police, and courts, yet there is no justice?
Christians often cite Jesus' condemnation of those that neglect justice, as a source of comfort--"heaping coals of fire upon their heads".
For those that Seek Vengeance: this is unsatisfactory, because this is an unfathomable reality--and does nothing to resolve the conflict--now.
For the Merciful: this is not satisfactory, because True Justice is not revenge, or retaliation.
True justice begins with Mercy, James 2:8, wherein those that hurt you have a true change of heart, seek peace, and pursue reconciliation--Justice, through mercy, where healing and restoration is found.
The precept of "Justice through Mercy", (Matt. 6:12-15), is the reason why Jesus' command to "not retaliate" is contextually found associated with his commandment to show mercy--to pray for, and love, and forgive enemies.
Matt. 6:14-15, NASB - “For if you forgive men their trespasses, your heavenly Father will also forgive you. 15 But if you do not forgive men their trespasses, neither will your Father forgive your trespasses.
From Matt. 18:23-35 - Those who forgive, are forgiven.
James 2:13, NASB - For judgment will be merciless to one who has shown no mercy; mercy triumphs over judgment.
It is a thought-provoking concept that Mercy, non-retaliation, can be the Justice, (and the judge), that brings true justice, conviction, rehabilitation, healing, and restoration.
The hard part, of course, is the mind boggling cost--and whether the Cross is considered sufficient.