Here is the passage in question (2 Thessalonians 2:1 - 8):

Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and [by] our gathering together unto him, that you be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand.
Let no man deceive you by any means: for [that day shall not come], except there come a falling away (gk. apostasia) first, and that man of sin be revealed, the son of perdition; who opposes and exalts himself above all that is called God, or that is worshipped; so that he as God sits in the temple of God, showing himself that he is God. Do you not remember, that, when I was yet with you, I told you these things?
And now you know what withholds that he might be revealed in his time. For the mystery of iniquity is already working: only he who now lets will let, until he be taken out of the way. And then shall that Wicked be revealed, whom the Lord shall consume with the spirit of his mouth, and shall destroy with the brightness of his coming:

1. There must first be "a falling away" (gk. "the apostasy") .
2. The Man of Sin must be revealed.

This article will be concerned only with the first event on this list.
Those who believe in a pre-tribulational rapture see here a reference to the general decline of christendom into apostasy. Paul's other epistle to Timothy ( 3:1 - 8) is often appealed to for a cross-reference to this falling away:
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This know also, that in the last days perilous times shall come. For men shall be lovers of their own selves, covetous, boasters, proud, blasphemers, disobedient to parents, unthankful, unholy...

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In the determining of what Paul meant in his use of "apostasia" in 2 Thessalonians 2:3 there are some things that should be kept in mind:

Who were Paul's listeners/readers and what would they understand by the term? Acts 17 says that Paul and Silas "came to Thessalonica, where there was a synagogue of the Jews: And Paul . . . reasoned with them out of the scriptures . . . And some of them believed, and joined themselves to Paul and Silas; and of the devout Greeks a great multitude, and of the chief women not a few."�
These Thessalonians, both the Jews and the Greeks who went to the synagogue, were well exposed to the Old Testament. The Septuagint, which they would be familiar with, always used the word "apostasia" to mean Jewish religious defection and abandonment.
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Here are the verses from their LXX (the Septuagint) of the Old Testament and Apocrypha:

In Joshua 22:22 the 2 and a half tribes defend their building of a memorial altar "Ed', saying:

"If we have transgressed before the Lord by apostasy, let him not deliver us this day." (Brenton)

2 Chronicles 29:19 and 33:19 speak of the "apostasy" of two evil kings, Ahaz and Mannaseh. In the first verse, Hezekiah cleanses the temple that Ahaz "polluted in his reign, in his apostasy". (Brenton)

Ahaz brought in the worship of pagan gods from neighboring lands. He drew the plans for a graven image he saw in Damascus and had one just like it put in the temple of the Lord. 33:19 mentions Mannaseh's "prayer also, and [how God] was intreated of him, and all his sin, and his trespass" (apostasia). (KJV)
What had this last king of Judah done? He worshipped the hosts of heaven (demons), made his sons pass through the fire and put carved images which he made in the temple of God.

Jeremiah 2:19 says of Israel:
"Thine own wickedness shall correct thee, and thy backslidings (apostasia) shall reprove thee..."

This and the previous verse show that the "apostasia" consisted� in Israel's trusting in foreign lands and in forsaking the true God.

1st Maccabees 1:54 - 2:15 relates the story of Antiochus Epiphanes (prototype to the Antichrist) his "setting up of the abomination of desolation" (1:54) and his wearing down and oppressing of the faithful Jews.
2:15 states "the king's officers, such as compelled the people to revolt (apostasia), came into the city Modein, to make them sacrifice". (Brenton) The sacrifice was the offering of a pig on the altar in devotion to Zeus Olympia.

Acts 21:21 mentions Paul's supposed leading of Jews everywhere to "forsake Moses" (lit. apostasy from Moses).