The authorship of 2 Thessalonians is hotly debated among students of the New Testament today. When considering the author of 2 Thessalonians from a Christian perspective, however, it seems first important to point out that the letter was either the product of Paul, or a complete forgery. It was common at the time to write a letter and attribute it to an authoritative person in order that it may gain authority quickly (this referred to as pseudepigraphy), however, the statement in 3:17, "I Paul write this in my own hand - the distinguishing mark in all my letters", is beyond the usual range of pseudepigraphers, claiming to come from the hand of the apostle himself. In such a case, Christians must consider how a forger could write such a Christian letter.
Those opposed to Pauline authorship, will use various evidences for their case. First, that the second letter to the Thessalonians uses much of the same language (and words) as the first letter to the Thessalonians, which implies that a forger to a large extent copied the themes and language of 1 Thessalonians while writing 2 Thessalonians; and it raises questions over the ability of one author (i.e., Paul) using the same language in two seemingly contradictory ways. For example, the use of Paul's labour in Thessalonica is used in both the letters. In the first, the author (probably Paul) stresses that he earned his own living in order not to be a burden to them (e.g. 2:9); in the second the author states that he earned his own living in order not to be a burden to any of them and to be an example to them, so that they would not be lazy (3:8-9). Opponents of Pauline authorship will cite this as an example of a forger copying Paul's authentic (first) letter to the Thessalonians, but adapting it to a different theme. However, Paul records the strain that 'earning his own living' caused on him (2:9), indicating to my mind that when Paul looked back on his days in Thessalonica to find examples from his own living that he could use to illustrate a point to the Thessalonians, the consuming effort of such continuous hard work and strain would have played large on his mind, understandably then a prominent memory from which to draw examples for teaching.
Similarly, the discussion of the day of the Lord in 2 Thessalonians (2:1-12) is often viewed as inconsistent with the teaching in 1 Thessalonians (and other Pauline writing), primarily as it focuses to a large extent on 'the man of lawlessness' that will come prior to the Day of Christ - elsewhere, Paul teaches that Christ will come 'suddenly, here the author (effectively) states that there will be signs first. Carson, Morris and Moo point out that the difference could be attributed to the appearance that originally, the Thessalonians needed teaching on the day of the Lord in general, and since the first letter had been misled to believe that the day had already come and so needed assurance of the true future coming of Christ.
It is also noticed that the letters seem to have a different tone - the first being warmer and more colourful; the second being colder and more formal (e.g. 1 Thess. 2:17, 1 Thess. 1:2 cf. 2 Thess. 1:3), thus indicating to some the probability of different authors. Guthrie, however, points out that "it is a fallacy to assume that any writer must always write in the same tone, since tone is very much a matter of mood which is in turn easily affected by prevailing circumstances". Guthrie also suggests that the first letter is 'warmer' as Paul had just received encouraging news about the Thessalonians from Timothy (2 Thess. 3:6); whereas the second reflects the apostle's perplexity at the Thessalonians being led astray (the circumstance which prompted the writing of the letter); with these circumstances explaining the mood of the letters (Guthrie, p.596). Carson, Morris and Moo also point out that the 'more colourful' passages in the first epistle regard the apostle defending himself (2:1-3:13) - a theme which does not occur in the second epistle.
It seems clear that it was Paul's practise to use an amanuensis (similar to a secretary) to write the majority of his letters, then towards the end wrote the personal greeting (and sometimes accompanying statements) in his own hand, which was his distinguishing mark. In Galatians he pointed out again that he wrote the last passage in his own hand (Gal. 6:11), as in 1 Corinthians (16:21) and Colossians (4:18). The fact that he doesn't specifically draw attention to this in other letters doesn't imply that they are forgeries, as the changed handwriting would have been obvious in an original copy. (Also, it is clear that this is not an uncommon practise when writing letters, as other letters with this format from the time have survived. See Morris, "TNTC: 1&2 Thess.", pp.151-2.)
It remains then, that there is no overriding evidence to assume that 2 Thessalonians is not authentically written by Paul - it has Pauline structure and language, and certainly claims to have come from his hand - therefore we can continue assuming Pauline authorship.
This must have been written soon after the first letter to the Thessalonians, placing it between 50-52 A.D., most likely while the Apostle was still in Corinth. Opponents of Pauline authorship are obliged to place it later, though not too late as it contains rather primitive theology in some cases.
Thessalonica ("Thess-al-on-eye-ka") was under Roman rule, and was known for the cult of Dionisius. This cult had much to do with sexuality and its symbol was the Phallus. It seems that overtones of this sexually provocative influence in the city forced Paul to write concerning sexual purity. (See 4:3-8).
Acts 17:1-9 records the beginning of Paul's ministry to the Thessalonians. He preached in the Synagogues, on three Sabbaths reasoning from the Scriptures. Several Jews, and "God-fearing Gentiles" were converted. But soon, hostile Jews rounded up a mob and laid charges against him, so he left for Berea (Acts 17:10-15). From Berea, Paul travelled to Athens (Acts 17:16-34) and then Corinth (Acts 18:1-18), from where he wrote the First Epistle to the Thessalonians, and most likely this one soon afterward.
It seems that there had been some belief spread in the Thessalonian Church suggesting that Christ had already returned. The Apostle uses this letter to make it clear that such has not yet occurred, and will not take place until "the Man of Lawlessness" is revealed (2:1-12). True to character, Paul also uses the opportunity to encourage the believers in their faith (1:2-12, 2:13-17), and give further teaching on the way to a holy life in Christ (3:6-15).