Apostasy (ἀποστασία)
Apostasy means “to stand away from.” The word is derived by taking the Greek preposition ἀπὸ (from) and adding it to ἵστημι (to stand) to make the meaning explicit. Tracing the etymology of the word from the Greek into English, we find that the English definition adds “from the faith”; expressing that an apostate is someone who has abandoned a religious system of belief. However, we must be cautious not to restrict the original sense to this English definition, as the original meaning requires the context to determine what exactly a person is departing from.
In 2 Thessalonians 2:3, the term “apostasy” is used to describe the departure, or "standing away," of the Church from the earth just before the beginning of the tribulation period, not a departure from the faith. In Acts 21:21, “apostasy” is used in the sense of departing from a religious system. Here, James is speaking with Paul as he tries to address the accusations of the Jews in Jerusalem, who have been told that Paul teaches the Jews among the Gentiles to forsake Moses. “And it has been reported to them concerning you, that you teach apostasy (standing away) from Moses, all the Jews among the Gentiles saying, do not circumcise their children nor walk in the customs” (Acts 21:21).
Apostasy is also used in the context of divorce, as it signifies a departure from the covenant made between a man and a woman (Mark 10:4). Thus, it is crucial to define from what exactly is being "stood away from" in any given context, as it could refer to a religious system, a marriage, or even a physical place.
In the letters to the Thessalonians, Paul is writing of the day of the Lord when we will gather together with Him, not concerning the faith. In 1 Thessalonians 4:16-17, Paul elaborates on what will occur during this time, describing how the Lord will descend from heaven, and those of the Church, both the dead in Christ and those alive, will be caught up to meet Him in the air. He encourages the Thessalonians not to be disturbed by false claims suggesting that the day of the Lord has already come. “Now I ask you, brethren, concerning the coming of our Lord Jesus Christ and our gathering together concerning Him, to not be quickly shaken from the mind neither be troubled whether through spirit or through word or through a letter as through us, as that the day of the Lord has happened (ἐνίστημι – in standing).” Then in 2 Thessalonians 2, Paul states that this day will not come until the departure of the Church has happened first. “Do not let someone deceive you according to any manner, because unless the departure (ἀποστασία – standing away from) comes first then the revelation of the man of lawlessness, the son of destruction.” 2 Thessalonians 2:1-3. Paul’s intention here is to reassure the believers not to be "quickly shaken in mind" or troubled by rumors coming by a spirit, spoken words, or a letter as though it has come from him, that suggest the day of the Lord has already occurred.
Until the Church meets Christ in the air, and therefore, the departure from the earth happens, the man of lawlessness will not be revealed. Since the man of lawlessness begins the tribulation period with the signing of the seven-year peace covenant with Israel, the Church must be removed from the earth before the tribulation period so that Satan is no longer restrained to raise up the man of lawlessness. Therefore, the “apostasy” in the context of 2 Thessalonians 2 is clearly the departure, or rapture, of the Church from the earth, not a departure of those on the earth from the Christian faith, because the removal of the Church is required before the man of lawlessness can be revealed, resulting in the Tribulation period coming upon the earth.