“…in flaming fire taking vengeance on those who do not know God, and on those who do not obey the gospel of our Lord Jesus Christ“
in flaming fire
Paul describes the eternal judgment on those without Christ as a judgment of “in fire of flame.”
Biblical vengeance punishes people based on what they rightly deserve. God gives justice to persecuted Christians by inflicting harm on their enemies. It is not a matter of emotional retaliation or revenge out of a sense of personal rejection, but a matter of justice.
on those who do not know God,
God will take vengeance on two groups of people: 1) those who do not know God, and 2) those who do not obey the gospel. There is no freedom from guilt by ignorance (Romans 1:18-21, 28-30). There is culpability because God revealed Himself to them in nature.
And on those who do not obey the gospel of our Lord Jesus Christ
There is a greater onus on this second group because they understood the implications of the gospel clearly. Clearly exposed to the light of revelation, they rebelled against it (John 3:19). They heard the gospel and rejected it.
There are many people even in evangelical churches who have heard the gospel but rejected it. This phrase refers to them as well as others who have had wide exposure to the gospel.
God always executes punishment with justice.
None of us likes to think about eternal retribution. We are like children who do not want to face the cost of our choices. Those who reject Christ will not be able to use a rationalization about hell as an excuse for rejecting Him.
“For the time has come for judgment to begin at the house of God; and if it begins with us first, what will be the end of those who do not obey the gospel of God?” (1 Peter 4:17).
God’s great challenge to you who have heard the gospel but reject it, is,
“Then the word of God spread, and the number of the disciples multiplied greatly in Jerusalem, and a great many of the priests were obedient to the faith” (Acts 6:7).
“But God be thanked that though you were slaves of sin, yet you obeyed from the heart that form of doctrine to which you were delivered” (Romans 6:17).