2 Thessalonians 1:7 And to you who are troubled rest with us, when the Lord Jesus shall be revealed from heaven with his mighty angels, 8 In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ:
The troubled Christians mentioned in these and surrounding verses where being persecuted for their faith. Christians continue to be persecuted in this world. Unless persecutors have certain criminal disorders their delusional self-justifying reasons have satisfied them enough to attack another human.
This passage teaches that their self-justification and subsequent actions are really a “token” of God’s present and future judgment of them. A person who commits a crime of hatred against and in the sight of the Judge of all the earth has willfully endangered himself. Judgment will come.
Vengeance, as used in this verse, can mean retaliation, vindicating justice, or punishment. In this case it is not retaliation. It is justice that vindicates the harmed person(s) and justly punishes the person(s) who harmed them.
Righteous vengeance is not bad or wrong. It is the result of a process involving evidence and a judge with the jurisdiction to hear and decide the case. In this situation the Judge is God, the judged are those who have persecuted His children, and punishment of the guilty is intense and justified.
It is clear that those who persecute Christians are the ones intended in this passage to be recipients of this righteous vengeance. Some modern translators have tried to lessen the intensity of the language, but, believe me, this is a harsh, brutal warning. But it is also clear that the persecutor can be freed from this deserved vengeance by obeying the Gospel. Obey means to repent, believe, and trust the Gospel. The writer of this passage, the Apostle Paul, had been a horrible persecutor of Christians, but he obeyed the Gospel and found forgiveness for all his sins including those committed against God’s children.