Odd Objection 1: Jesus would never call Christians "evil." Hence, any designation of a person that uses the words "evil" or "unjust" must be talking about an unbeliever.
Let's see if this is true.
Matthew 7:11: "If you who are evil, therefore, know how to give good gifts to your children, how much more will your Father who is in heaven give what is good to those who ask Him!"
Luke 11:13: "If you who are evil, therefore, know how to give good gifts to your children, how much more will your heavenly Father give the Holy Spirit to those who ask Him?"
Who is the "you"? Unbelievers? Is God granting what is good and the Holy Spirit to unbelievers who apparently are asking for things in faith? Obviously, this objection is negated right here by Jesus Himself. Believers act wickedly all of the time. They are evil people who are being redeemed, but that redemption has not found its fulfillment yet, so they still do evil things and are identified as evil people. They can also be righteous and good in so far as they are walking in the Spirit of God, but the idea that Jesus would never call His people "evil" is just ignorance of what Jesus actually said.
Odd Objection 2: Christians would never slander one another and persecute one another with insults. That sounds more like what unbelievers would do to Christians.
Again, this objection is like the first. If it was a true statement that Christians don't slander one another and persecute one another in terms of insults and gossip, then we should not see any need for the New Testament to warn Christians to stop doing it. Do we see the New Testament warning them to stop this? Yep.
By the way, that is said by Paul about the Jewish-Gentile conflict in the early church. Jewish Christians were slandering Gentile Christians and vice versa.
2 Corinthians 12:20: "For I am afraid that perhaps when I come I may find you to be not what I wish and may be found by you to be not what you wish; that perhaps there will be strife, jealousy, angry tempers, disputes, slanders, gossip, arrogance, disturbances . . ."
James 4:11-12: "Brothers, do not slander one another. Anyone who speaks against his brother or criticizes him speaks against the law and criticizes it. And if you are a critique of the law you are not one who practices it, but criticizes it. There is only one Lawgiver and Judge, the One who is able to save and destroy. But who are you to judge your neighbor?…"
By the way, again, speaking to Jewish Christians about how they treat, perhaps, other Jewish Christians.
1 Corinthians 6:6-7: "Instead, one brother goes to court with another and this in front of unbelievers! The very fact that you have lawsuits among you means you are thoroughly defeated already. Why not rather be wronged? Why not rather be cheated?…"
There is actually a lot of this going on in the New Testament. It even has its precedent in the Old.
"Whoever secretly slanders his neighbor, him I will destroy" (Ps 101:5).
But the really odd thing about this objection is that the very texts that we're talking about refer, in the context, to this going on within the covenant community.
Christ just warned them that they would be judged if they do not settle their disputes with one another, and that slandering one another by calling one another names (i.e., insulting a brother) is a crime punishable by hell (5:21-26).
The fact that both divorce and adultery are brought up shows that the disputes in question are internal to the covenant. It is simply odd to suggest that the one who slanders another Christian would unlikely be another Christian. Indeed, it seems that the one who slanders another Christian, especially in the context of this Jewish-Gentile debate, would most likely be a Christian.
In fact, in Luke, it is clear that he is addressing the visible covenant community and its conflicts as something that has always happened within the visible community of God. Those who claim to be of God always persecute and insult those who are truly of God "just like their fathers used to treat the prophets." Their pagan fathers? Are we talking about unbelievers or people who profess to be a part of the visible covenant community? The latter seems clear.
Luke's insults have to do with having a bad name because of one's faithfulness to Christ, and being excluded, in the context of Luke, for hanging out with sinners and outcasts (i.e., the lowly) who are now in the Christian community. It has nothing to do with persecution from unbelievers for believing in Jesus, but everything to do with eating and drinking with people of low reputation that have now come into the kingdom. The "evil people" here are said to be believers who actually have an "evil name," i.e., a bad reputation for doing what Christ did with poor and outcast Christians (i.e., associating with them).
So what is all of this? It is the statement of the problem. It is clearly a description of the conflicts that are going on in the church. What, then, is Matthew 5:38-48 and Luke 6:27-36? It is the solution. Christ commands His followers who have an ear to hear to not respond to other Christians who are mistreating them, insulting them, wronging them in some way, taking from them, etc. with an eye for an eye, but rather, because justice has been served on the cross, they can show mercy and let it go. They can respond to one another in love and kindness, even though these so-called Christians have not shown love, but only hate, toward them.
Finally, Odd Objection 3: "Enemy cannot refer to a believer, but only an unbeliever. I just cannot imagine that "enemy" would ever refer to someone in the visible covenant community."
I can understand this objection. It likely has to do with the connotations of our word "enemy." We often think it has something to do with someone who wants to kill us or something. However, the word really just expresses a person who is at odds with another person. Someone who is hostile toward another is an enemy. Someone who is bitter toward another is an enemy. Someone who slanders another is an enemy. Someone who just doesn't like you is an enemy. If you notice, "enemy" is often contrasted with "friend." There are people with whom we get along within the covenant community and people we do not. There are Christians who are bitter, slander, gossip, backbite, etc. against other Christians. The idea that one has no opposition or enmity with other Christians is really odd in light of the New Testament teaching that this is actually a constant problem in the church.
For instance, Herod and Pilate are said to be enemies who become friends in the Gospel of Luke. They're both pagans, but they are not out to kill one another. They don't really do much against one another. They just don't like one another. They don't get along. Hence, they are called enemies/opponents, etc.
Luke even tries to describe that this is what he means by "enemy" when he says that these are people who don't love you, but instead hate you. Love and hate can also be misconstrued, but the idea seems to be that some in the Christian community have a preference for their friends in terms of giving and lending, and exclude those who are not their friends from such kingdom benefits. Likewise, it was common for those in power, even in the covenant community (as is evident in the OT and in James) to take from those in lower positions, especially if they felt those in lower positions who worked for them did not do a perfect job for them.
The enemy is also, as discussed before, one who has brings his brother to court and creates division between them over disagreements.
In Psalm 127:5, it states, "Blessed is the man who has his quiver full of them. He shall not be put to shame when he speaks with his enemies in the gate."
The gate is a place of judgment. It was the court in ancient Israel. The enemy is not a foreign enemy, but a fellow covenant member who is bringing charges against him.
Likewise, throughout the OT, one can see enemies as foreign enemies, but also see them as within the covenant. David's greatest enemy, some might say, was actually the Anointed King of the covenant community, Saul. Saul is called David's enemy numerous times, showing that the designation "enemy" does not carry with it any inherent connotation of someone who is outside the covenant. Indeed, according to the contexts of Matthew and Luke, these "enemies" are within the covenant. The same goes for Romans 12, which I've discussed before.
In essence, the literary contexts are about how one group of Christians treats another group of Christians, the immediate context is about how one group of Christians treats another group of Christians, and the words cannot, should not, and do not, negate that context. Instead, their referents must be found in those contexts and not outside of them. When we do that, we see that both Matthew and Luke apply Jesus' original words, whatever they may have been (although they were likely also words that related to conflicts within the covenant community, as this is consistent with conflicts within the community throughout the OT), to deal with internal conflicts within the covenant community.
To say otherwise is to attempt to place unwarranted nuances to words that they do not inherently have, and then bend the entire context to meet those unwarranted nuances. Such is the stuff of eisegesis, not exegesis.