Must a follower of Christ forgive those who mistreat them? The short answer is yes, Christians are not permitted to hold grudges. We are called to forgive, even 7 times 70 times. Mathew 18:2-22 Then Peter came up and said to him, "Lord, how often will my brother sin against me, and I forgive him? As many as seven times?" Jesus said to him, "I do not say to you seven times, but seventy-seven times.
Simple as that may sound, how does forgiving someone work when contact with that person always leads to heartache or even physical pain? Are we simply called to allow ourselves to be emotional or physical punching bags for others? This is not so easily discerned through the clear teaching of scripture and we are often given bad advice by well meaning people who may have never experienced a toxic relationship.
Interestingly, the only implicit connection to forgiving and forgetting are in relation to God forgetting our transgressions. Isaiah 43:25 "I, I am he who blots out your transgressions for my own sake, and I will not remember your sins. We are never commanded to forgive and forget, we are commanded to forgive. Toxic relationships are such when a person repeatedly brings harm and has established a pattern and reputation for doing so. The best thing to do is to remove oneself from the relationship, sometimes permanently. In scripture, we see Jesus removing Himself from a dangerous situation as the people of His hometown attempt to throw Him off a cliff. Luke 4:29-30 And they rose up and drove him out of the town and brought him to the brow of the hill on which their town was built, so that they could throw him down the cliff. But passing through their midst, he went away. Also, after Jesus was crucified by the joint effort of the Jewish and Roman leaders, He called on the Father to forgive them, yet, after the resurrection, we do not see Jesus reaching out to the Roman or Jewish leaders, it would appear that He severed all ties to them, having spent 40 days going in and out among His followers. Jesus taught us to turn the other cheek, rather than retaliate. Jesus also showed us that we can remove ourselves from a dangerous situation and that we can forgive and love others with out maintaining a personal relationship.
Forgiveness is non-negotiable, but placing ourselves in danger is not necessary. Scripture is clear that there are times when we are to sever ties or disfellowship with others. Jesus gave us explicit instructions about breaking fellowship. Matthew 18:17 If he refuses to listen to them, tell it to the church. And if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector. Later, the Apostle Paul gives us instructions concerning disfellowship, making a marked distinction between believers and unbelievers. According to the following passage, there is good reason to break fellowship, with believers, for the sake of the person in sin. 1Corinthians 5:5&9-13 you are to deliver this man to Satan for the destruction of the flesh, so that his spirit may be saved in the day of the Lord... I wrote to you in my letter not to associate with sexually immoral people— not at all meaning the sexually immoral of this world, or the greedy and swindlers, or idolaters, since then you would need to go out of the world. But now I am writing to you not to associate with anyone who bears the name of brother if he is guilty of sexual immorality or greed, or is an idolater, reviler, drunkard, or swindler—not even to eat with such a one. For what have I to do with judging outsiders? Is it not those inside the church whom you are to judge? God judges those outside. "Purge the evil person from among you."
Though we are permitted, even commanded, by scripture, to break fellowship, we must be careful it is with much prayer and counsel and for the sake of restoration, not necessarily personal, but spiritual on the part of the offending party. We can forgive and love from a distance. We can pray and hope for the good of people we with whom we no longer associate. Sometimes there can also be personal restoration after true and proven repentance. The point is, ending toxic relationships is Biblical and sometimes necessary.