1. But relative to the coming of our Lord Jesus Christ, the Messiah, and our gathering together to meet Him, we beg you, brethren,
2. Not to allow your minds to be quickly unsettled or disturbed or kept excited or alarmed, whether it be by some pretended revelation of the Spirit or by word or by letter alleged to be from us, to the effect that the day of the Lord has already arrived and is here.
3. Let no one deceive or beguile you in any way, for that day will not come except the apostasy comes first—that is, unless the predicted great falling away of those who have professed to be Christians has come—and the man of lawlessness (sin) is revealed, who is the son of doom (of perdition). Amplified Bible

Paul is writing concerning the return of Jesus Christ to the earth and our gathering together to meet Him.  He says not to be quickly shaken in mind.  And not to be troubled by supposed revelations from the Spirit or by an alleged word or letter from Paul & Co., that would say the day of the Lord has already begun.

This should be a warning to us not to take everything that people say is from God to be from God.  We need to check everything out with the Word of God and the Spirit of God within us.

He goes on to say not to let anyone deceive you because that day will not come until there is a falling away from the truth.  The Greek word for falling is ‘apostasia’ meaning defection from truth.  This falling away is amongst those who are within the church—those who love and know the truth and fall away from it.  These may not necessarily leave the church, but leave the truth.

The other thing that has to happen before the day of the Lord is that the ‘man of sin’, is revealed.  He goes on and describes him in verse 4.  He’s one who exalts himself above God and claims to be God.  This person will be revealed or uncovered prior to the day of the Lord.

I will not be shaken or troubled by reports that are not true.  I’ll not be deceived by false reports concerning the coming of our Lord Jesus.  I will remain in an attitude of trust in Him and love of the Truth.