2 Thessalonians 2:1 & 2

"Concerning the coming of our Lord Jesus Christ and our being gathered to him, we ask you, brothers, not to become easily unsettled or alarmed by some prophecy, report or letter supposed to have come from us, saying that the day of the Lord has already come." NIV translation

In chapter 1, Paul had tried to reassure the believers that persecution was normal for followers of Christ. People had confused this persecution with what is described as the great tribulation period or the day of the Lord. He had taught them in his first letter that the believers would be caught up (raptured) when Jesus came and so the people were wondering why they were still there and not with Jesus.

2 Thessalonians 2:3

"Don't let anyone deceive you in any way, for that day will not come until the rebellion occurs and the man of lawlessness is revealed, the man doomed to destruction." NIV translation

Paul gets right down to it and meets this false teaching head on by saying it has not come and will not come until two things occur. The first is the rebellion and the Greek word refers to apostasy or departure from the faith. This first sign speaks of the complete turning away from faith in Jesus Christ as the Son of God. It also speaks to the departure of the true believers from this earth in the rapture. We are called to be the light of the world and the day of the Lord (like any other Hebrew day) begins in darkness. Therefore, we will be gone before this darkness and total rebellion happens. The second sign will be the revealing of the "man of lawlessness" also known as the beast (see Revelation 13).