What is the day of Christ?
The day of Christ is the rapture—the outcalling of the church, when Christ calls the body of Christ unto Himself. It is distinct from the day of the Lord, which includes the tribulation, the second coming, and the kingdom. Scripture says that day will not come except a falling away first.
Les explains that the day of Christ refers to the outcalling of the church, the rapture, when Christ will call the body of Christ unto Himself. He contrasts it with the day of the Lord, the day of Jehovah, the day of God, which is the period including the tribulation, the second coming, and the setting up of the kingdom. In verse 3, Les teaches that the day of Christ shall not come except there come a falling away first. That word in the Greek is apostasia, from which we get apostasy, meaning a turning away from the revealed truth of the Word of God. He notes that many churchgoers do not know what apostasy is, and asks whether the churches for the most part have apostasized today.
That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand.