Apostasy means “to stand away from”. It is created by taking the Greek preposition ἀπὸ (from) and adding it to ἵστημι (to stand). Unfortunately, often times the English definition of this word is implied to Scripture, rather than its proper translated meaning.
This is especially prevalent in 2 Thessalonians where some imply that before the coming of Christ there will be a departure from the faith. However, what is being departed from must be defined by context because apostasy (ἀποστασία) only means “to stand away from”. The English definition adds “from the faith”. This is one of the issues that is caused by not translating a word. The transliterated word begins to take on a definition of its own that does not reflect the meaning of the original word. This new definition is then applied to the original, perverting its meaning.
Apostasy (ἀποστασία) is used for departing from a religious system, but what is being departed from is defined by the context. In Acts 21 James is speaking with Paul as he tries to outwit the Jews in Jerusalem because they 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 departure from Moses, all the Jews according to the Gentiles saying, do not circumcise their children nor walk in the customs – Acts 21:21.
However, apostasy is also used of departing from the earth. In 2 Thessalonians, Paul is writing of the day of the Lord when we will gather together with Him. In 1 Thessalonians 4:16-17, where Paul gives more details on what happens at this time, our meeting is in the air.
Now I request 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). Do not let someone deceive you according to any manner, because unless the departure (ἀποστασία) comes first also the revelation of the man of lawlessness, the son of destruction. 2 Thessalonians 2:1-3.
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 is removed from the earth before the tribulation period. The “apostasy” in context here in 2 Thessalonians is clearly the departure from the earth, not the faith.