Paul tells us that it is through much tribulation that we enter the Kingdom of God (Acts 14:22). That’s a fact; there is no other way to enter. Jesus said that the Kingdom of God (heaven) comes to those who are persecuted for righteousness (Matthew 5:10). While men seek to enter the Kingdom of God (Luke 16:16), religious leaders prevent them through threats (cp. John 9:20-23; Acts 4:21) and other means (cp. Acts 5:40). Yet, those who persist succeed (Acts 5:41; 14:22). In 2Thessalonians 1:5 Paul told his readers that the very fact that they were suffering persecution over their faithfulness to God was a visible sign that the Lord had received them into his Kingdom. Nevertheless, not only is this true for the righteous, but their unjust persecution demands justice from a righteous God.
Therefore, the Lord will repay such things by bringing destruction upon their persecutors (2Thessalonians 1:6). In other words, persecution is a certain sign of salvation to believers, but of impending judgment upon the evil doers (cp. Philippians 1:28).
Some have thought that persecution brings salvation to the believer, but this misses the mark. Only Christ is able to offer salvation to anyone. Only his blood saves. Persecution is merely a sign of what has already occurred. The believer has already been saved and the world responds with persecution, because the world is unable to accept anything that is not of this world. It fears what it cannot control—what it cannot see, hear, touch, taste or smell (the five gates of knowledge for the flesh). Whatsoever is not of the world, will be feared and persecuted by the world. Therefore, persecution is a sign both of salvation on the part of the believer and of judgment on the part of the persecutor (2Thessalonians 1:5-6)
Paul then encourages the persecuted believers at Thessalonica by telling them to rest with him, because they know how he was ill-treated at Thessalonica and even unjustly expelled from their city by their local authorities (2Thessalonians 1:7). Nevertheless, he didn’t seek to retaliate or seek revenge upon his persecutors. Rather he waited for the Lord to vindicate him. The Lord simply cannot sit by forever without judging evil. The Scriptures attest to this well enough, so Paul’s advice for the believers at Thessalonica was to wait for that to occur; don’t retaliate; don’t nourish a revengeful heart, but wait; wait with him (Paul) for he hasn’t retaliated against his persecutors. Wait for the Lord to judge the evil done.
Paul, together with all the New Covenant writers, have preached that Jesus would return within the lifetime of the then present generation of believers (2Thessalonians 2:1; cp. Acts 1:11; James 5:7-8; 1Peter 1:7, 10-12; 1John 3:2; Jude 1:14-15). Jesus, himself, said as much (Matthew 16:27-28; 23:34-36; 24:29-31, 34). Therefore, when Paul told the believers at Thessalonica that they should wait for the Lord, Jesus, to be revealed from the heavens for their vindication, he was speaking of the Lord’s Second Coming (2Thessalonians 1:7).
The Lord would be revealed in a flaming fire (2Thessalonians 1:8; cp. Exodus 3:2; Acts 7:30), taking vengeance upon those who are ignorant of God (gentiles) and of those who reject the Gospel (the Jews). Yet this doesn’t mean the Lord’s coming would be a literal return for all to see with their physical eyes, for the prophet also says:
Who gave Jacob for a spoil, and Israel to the robbers? did not the LORD, he against whom we have sinned? for they would not walk in his ways, neither were they obedient unto his law. Therefore he hath poured upon him the fury of his anger, and the strength of battle: and it hath set him on fire round about, yet he knew not; and it burned him, yet he laid it not to heart. (Isaiah 42:24-25; emphasis mine)
The mention of fire is a metaphor of a serious reality. It was the Lord judging evil in the day when he came to judge his people, Israel. Fire is a symbol of judgment, as the Scripture says: the Lord is a consuming fire (Hebrews 12:29), and a fire goes before him (Psalm 97:3). Peter even prophesied for the days of his generation in the first century AD that the whole earth would “melt with fervent heat and all the works therein would be burned up (2Peter 3:10). Nevertheless, Peter wasn’t speaking of a literal fire that would destroy the earth, but, rather, he spoke of the time when the earth (the Jewish state) would be judged and the Old Covenant would come to an end. This was the time, concerning which, the Lord spoke (Matthew 16:27-28), and was the time Paul said that he and all the persecuted believers needed to await (2Thessalonians 1:7-8).