I think you would enjoy the book - turns out there is more to the Greek than meets the eye. I had never looked into it till recently and in answer to your question, "departure" has more than one interpretation...
Based on the use of this word in the New Testament as well as the Old Testament Septuagint, context, an apostate is someone who turned away from obedience to God.
Please read the link to the scriptures that use this word.
In addition, the coming of the Lord and the gathering together to Him, is the resurrection and rapture, not the “falling away” as described in
1 Thessalonians 4:15-17
- we who are alive and remain shall be caught up together with them
17 Then we who are alive and remain shall be caught up together with them in the clouds to meet the Lord in the air. And thus we shall always be with the Lord.
1 Thessalonians 4:17
The living and dead will be gathered to Him at His coming.
The coming of the Lord won’t happen until after the man of sin is revealed.
The man of sin, is actually destroyed by the brightness of His coming.
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 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.
2 Thessalonians 2:1-4
The false messiah comes first, works his deception and deceives people into following and worshipping him, whereby they depart from the faith in Christ, the true Messiah.
Then at the coming of the Lord, the false messiah will be destroyed by Christ Himself.
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.
2 Thessalonians 2:8
If the True Messiah comes first, who would follow the false messiah?
JLB