In his first Thessalonian letter, Paul reminded the church of what he’d taught them in person concerning the return of Jesus in the air (1 Thess 4:13-18) and the Day of the Lord (1 Thess 5:1-11). But rumors from false teachers created discouragement, fear, and confusion that their trials and persecutions were proof Jesus had come and they’d been left behind!
This chapter has been labelled “difficult”, but it’s easy to understand when we take it literally, just as Paul wrote it.

I. The Tenet (2 Thess 2:1). Paul links two subjects in one event: (1) the coming of Jesus. Coming (parousia) is used 24 times in the New Testament and in every instance means physical presence (1 Thess 2:19; 4:15).
(2) our gathering (episunagoges) together to Him. This word is used only two other times in the New Testament, first of the gathering of believers on earth (Heb 10:25); and second of the gathering of the living and dead to Jesus in the air (1 Thess 4:13-18).

II. The Trouble (2 Thess 2:2). Only months after writing his first letter to the Thessalonians, troubling rumors quickly (soon) spread that the Day of Christ [better, Day of the Lord] had already come. The Day of the Lord is the same as the coming of Jesus (2 Thess 2:1). These rumors had shaken (saleuthenai) and troubled (throeo) the congregation. Saleuthenai describes a sudden rocking back and forth that continues, like a boat tossing in a storm or the shaking of an earthquake (Acts 4:31; 16:26). Jesus used the word throeo in the Olivet Discourse (Mt 24:6; Mk 13:7), which literally means to cry aloud as in pain or shock; to be frightened, scared, panicked. The rumors taught by false teachers poisoned the composure and poise of the believer’s minds.

III. The Truth (2 Thess 2:2-3). The false rumors came in three forms: by a spirit, by word, and by letter. The word spirit refers to a prophetic revelation. The prophetic words were important in the early church because the New Testament had not yet been written, or what was written wasn’t widely circulated or compiled. Biblical truth was built on the Old Testament and expanded on by ecstatic prophetic messages. Two or three prophets were permitted to speak in the assembly, but the messages had to be judged by others as to accuracy (1 Cor 14:26-33; 1 Thess 5:20-21).
"Word" (logos) describes a verbal teaching, especially that from the apostles (Acts 2:42; Eph 2:20; 3:5). The letter (epistle) was written instruction. Today we call it the New Testament. False teachers were claiming equality with Scripture!
The New Testament contains all we need to know for salvation and godly living. God is not revealing new truth not already found in the Bible. Once the foundation of the church was laid in Scripture, new revelation ceased; what believers need today is to preach the word (2 Tim 4:2) and obey the Word (2 Tim 3:14-17; 2 Pet 1:19-21); not hear a new word.
Paul denied the Day of the Lord had come, and the new revelation was not what he’d taught — that’s why they were troubled! Some today say that the Church will go through the Day of the Lord or the Great Tribulation described by Jesus (Mt 24). They even say it will be a good thing for the Church because Jesus will protect His people. But Paul denied the Day of the Lord had come. If it was a good thing, why was the Thessalonian church troubled and shaken? They should have been rejoicing.
Paul urged the believers not to be deceived (exapatao), to be led astray. False prophecy, false teaching, and false letters led away from truth, so believers today must know the Scriptures well to avoid their own deception.