This epistle might be referred to as 3rd Thessalonians, but not really. There may have been a forgery supposedly from Paul but actually from an imposter, and that might have been why the Thessalonian church was confused on certain matters. This was alluded to in Ch. 2:2 (“not by letter as by us”).
This letter was written by Paul shortly after the first letter to the Thessalonian saints; both around A.D. 51.
Theme: The Day of the LORD (This phrase is also seen in Joel 1:15; 2 Ptr. 3:10 and Rev. 19:19 and elsewhere). This phrase refers to that period of time when God actively and openly begins to deal with the people of the Earth in judgment, beginning with the Jew and then the Gentile as portrayed in Revelation Ch. 6 – 19.
It begins with (1) the translation of the church or rapture (“bride of Christ”) from Earth to Heaven and culminates at (2) the Great White Throne Judgment with the 70th week of Daniel (“time of Jacob's trouble” Jer. 30:7) in which the world will see the 7 year reign of anti-Christ or “the man of sin” (2 Thess. 2:3) occurring between those two points.