Is the Greek, apostasia equivalent to the modern definition of the English, apostasy?
While in the power of the Holy Spirit, the apostle Paul wrote his second letter to the Thessalonian church with both encouragement (2 Thess. 2:1-7) and warning (vs. 8-12).
The encouragement is the fact that we are going to be "gathered together" and "depart" the earth before the "revealing of the lawless one".
"He who now restrains" (vs. 7) is the Holy Spirit Who not only holds back the anti-christ, but "gathers" or causes “physical departure” (Greek noun = apostasia) of “something” (vs. 3). That “something” is the Holy Spirit which would logically include “our gathering together unto Him”.
The warning is that there will be “condemnation” for those who "did not receive the love of the truth, that they might be saved".
The reasons I believe that the Greek noun 'apostasia' is just a very simple word for "departure" in a physical sense is because of the grammar, syntax and word-structure within the context. It begins in verse 1 with "gathering together unto Him". In verse 3 we see the Greek 'apostasia' translated in English as "falling away", and in verse 7 we see the expression, "taken out of the way". Each phrase implies a "physical movement" of some kind.
Also, the Greek verb for apostasia is the word aphistēmi which occurs 16 times in 15 separate verses of the KJV Bible. (https://www.blueletterbible.org/lang/lexicon/lexicon.cfm?strongs=G868&t=KJV). Almost all of these contexts imply a physical departure or physical separation away from something.
And finally, there is collaborative evidence of 'apostasia' being a physical departure if we respect the Spiritual insight of both William Tyndale and the Geneva Bible linguists. They each used the word "departing" when translating the Greek noun 'apostasia' in 2 Thess. 2:3.
Something to also consider is the unintended progression of certain words which have changed their meaning over many generations.
One word in particular is Apocalypse. A list of synonyms from Wiktionary for the word, Apocalypse include - Armageddon, doomsday, Ragnarok, end times, eschaton.
A common use (or misuse) of "apocalypse" is a term that we have all heard - "zombie apocalypse". When we hear it, we envision mass destruction, mayhem and terror from a ragged horde of "the living dead".
Yet, the English word Apocalypse is taken from the Greek word, apokalypsis. This is the name given to the final book of the New Testament where we read in chapter one, verse one, “The Revelation of Jesus Christ, which God gave unto him, to show unto his servants things which must shortly come to pass.”
The definition of the word apokalypsis is described with synonyms and phrases such as, revelation, be revealed, manifestation, coming, appearing, a disclosure of truth, concerning things before unknown, and unveiling.
The original definition of apokalypsis is nothing like the modern description of mass destruction, mayhem and terror.
In the same way, we see another unintended progression of the modern English word translated as, Apostasy taken from the Greek word, apostasia.
In 2 Thessalonians 2:3, the Greek noun apostasia is rendered in the NIV Bible as - "for that day will not come until the rebellion occurs".
In a similar way, the ESV writes it as, "For that day will not come, unless the rebellion comes first".
The NASB makes a more literal attempt by showing it as, "for it will not come unless the apostasy comes first".
And finally, the Amplified Bible makes a wild, presumptuous description with, "for that day will not come unless the apostasy comes first [that is, the great rebellion, the abandonment of the faith by professed Christians]".
The modern definition of apostasy according to Merriam-Webster is –
1: an act of refusing to continue to follow, obey, or recognize a religious faith
2: abandonment of a previous loyalty: DEFECTION
So therefore, using the term rebellion does sound like a correct interpretation. Yet, as introduced above, it was originally taken from the Greek noun apostasia; with the Greek verb being aphistēmi. This verb has been demonstrated to imply a physical departure or physical separation away from something when used in context.
Read the following passage with the “pre-trib rapture” in mind and see if you agree that the context for a physical departure (rapture) should be made.
2 Thess. 2:1-12 (NKJV), "Now, brethren, concerning the coming of our Lord Jesus Christ and our gathering together to Him, we ask you, 2 not to be soon shaken in mind or troubled, either by spirit or by word or by letter, as if from us, as though the day of Christ had come. 3 Let no one deceive you by any means; for that Day will not come unless the falling away/departure (apostasia) comes first, and the man of sin is revealed, the son of perdition, 4 who opposes and exalts himself above all that is called God or that is worshiped, so that he sits as God in the temple of God, showing himself that he is God. 5 Do you not remember that when I was still with you I told you these things? 6 And now you know what is restraining, that he may be revealed in his own time. 7 For the mystery of lawlessness is already at work; only 'He who now restrains' will do so until He is taken out of the way. 8 And then the lawless one will be revealed whom the Lord will consume with the breath of His mouth and destroy with the brightness of His coming. 9 The coming of the lawless one is according to the working of Satan, with all power, signs, and lying wonders, 10 and with all unrighteous deception among those who perish, because they did not receive the love of the truth, that they might be saved. 11 And for this reason God will send them strong delusion, that they should believe the lie, 12 that they all may be condemned who did not believe the truth but had pleasure in unrighteousness."