What does 2 Thessalonians 2:1 mean?

In this verse Paul addresses his reader as "brothers." He was confident that they had believed on Jesus as their Savior and had become members of God's family. God was their Father, and they and Paul were spiritual brothers. The brothers at Thessalonica had received Paul's teaching about the coming of Christ, in the air, to take Christians away from the earth to be with Him before the day of the Lord (1 Thessalonians 1:10; 4:13–17; 5:1–11).

Later, it seems, some false teaching about the end times had crept into the church. Perhaps itinerant false teachers had brought this teaching. Maybe the church had received a counterfeit letter bearing Paul's name that contained erroneous teaching (2 Thessalonians 2:2). Regardless of the source, this false teaching had confused some of the believers. They misinterpreted their severe suffering as evidence that they had entered the day of the Lord, the tribulation. Paul, therefore, begins a significant explanation concerning the Lord's return and the gathering of Christians to Him.