The apostle in addressing the saints spoke of the coming of our Lord Jesus Christ and our gathering, at the rapture, to Him. They were not to be shaken in mind, or troubled by the letter they had received, as if it were from the apostle (no doubt a forged letter with the apostle's name on it) leading them to believe that the day of Christ (Lord) was at hand. This was an error that had come in to trouble them.
The gathering together to Christ (verse 1) could not be the day of the Lord. It could not have yet come, because Christians were not gathered to the Lord in heaven; they were to come with Him at the day of the Lord.
Judgment could not be exercised because the wicked one had not yet appeared. Iniquity had to reach its height before the judgment of the day of the Lord would come. The saints had to be in the same position as Christ, to be gathered together unto Him, before He could manifest Himself in glory to those outside. Christ would publicly honor His saints in the sight of the rebels at the time of judgment.
The man of sin will appear in full rebellion, the man of the earth, in hatred to God and His saints. Generally there will be a renunciation of faith. The man of sin will assume the place of God in His temple and will take a place above all that is called God. His character here is anti-religious. The secular power is not mentioned here, although the two are inseparable in evil. He will come as the unbridled man, the lawless one, the man of Satan, against the Man of God. He will be judged by the Man Christ Jesus from heaven. As long as the Church is on earth these things could not happen; the man of sin could not come forth.
It is those who received not the love of the truth who will be judged. Many had seen and known the truth but did not love it, showing by their ways in minding earthly things that they were enemies of the cross of Christ. This enemy of truth, the man of sin, is the false Jewish Messiah, king, and prophet. He also will make fire come down from heaven, as Elias did to prove who was God.
The apostle prayed that God and the Lord Jesus Christ Himself who has given everlasting consolation and good hope through grace would "comfort your hearts and establish you in every good word and work."