As we mentioned in comments on First Thessalonians, many today deny that Paul wrote Second Thessalonians. There are two kinds of evidence for authorship, internal and external. Internal consists in things said or implied in the work itself; external consists in statements by early writers. There are several statements of early writers, saying that 2 Thessalonians is by Paul: St. Irenaeus (c.140 - c.202) who had listened to St. Polycarp tell his recollections of the preaching of St. John, quotes from both 1 Thessalonians and 2 Thessalonians as Paul's (PG 7.1061 and 1138). Similarly Tertullian (c.160 - 222 or later) and St. Epiphanius (c.315-403) quote both Epistles as Paul's. Some try to disregard all this early evidence by merely pointing to 1 Thessalonians 4:3-18, and claiming that shows Paul expected to be alive at the end. But we saw the passage can very easily be taken as not implying any belief on the part of Paul that he would live to the end.

Paul seems to have written 2 Thessalonians soon after 1 Thessalonians, after hearing that many there were upset, thinking the end was very soon, and even quitting work. Paul writes to correct this false notion, chiefly by pointing out, in chapter 2, that two signs must come before the end -- the antichrist and the great apostasy. But neither was in sight at the time.