Hello CV5,
I guess that we would be in disagreement here. Though departing from the law of Moses and turning to Christ and the new covenant of salvation by grace through faith is correct, that is not what Acts 21:21 is focused on. We must focus in on the meaning of the word apostasia itself, which is used in regards to departing from the teaching of Moses or in our case, departing or turning away from our faith in Christ. Below is the definition of apostasia, as well as the other translated words used to describe it:
Strong's Concordance
apostasia: defection, revolt
Original Word: ἀποστασία, ας, ἡ
Part of Speech: Noun, Feminine
Transliteration: apostasia
Phonetic Spelling: (ap-os-tas-ee'-ah)
Definition: defection, revolt
Usage: defection, apostasy, revolt.
HELPS Word-studies
646 apostasía (from
868 /aphístēmi, "leave, depart," which is derived from
575 /apó, "away from" and
2476 /histémi, "stand") – properly, departure (implying
desertion);
apostasy – literally, "a leaving, from a previous
standing."
The reason why I bring this up, is because we have those who try to make the translated word 'departure' as referring to departing up to meet the Lord in the air. However the word cannot be used in that way.
For many, the problem lies in not recognizing the words " the coming of our Lord and our being gathered to Him" in verse 1, vs "The Day of the Lord" mentioned in verse 2 and as "that day" in verse 3.
"The coming of our Lord and our being gathered to Him" is obviously speaking of the gathering of the church. Where "The Day of the Lord" is in reference to God's coming wrath, which follows the gathering of the church.
There were false teachers in Thessalonica proclaiming that The Day of the Lord, the time of God's wrath had already come. So they wrote to Paul who knew his teaching regarding the gathering of believers and the wrath to follow, basically saying to Paul, "Hey Paul, there are people here saying that the Day of the Lord has already come, so how come we haven't been caught up to the Lord according to your teaching. The Thessalonian's were concerned that because they hadn't been caught up, that they had entered into the Day of the Lord, the time of God's wrath. Paul then comforts them, saying that "The Day of the Lord" will not come until the apostasy takes place and the man of lawlessness is revealed, which had not taken place and still has not taken place.