the Greek  word translated "falling away" means  ""the departure""; primarily  in the sense of the removal of the church.  It also refers to the great Laodicean church of the last days.
 
In modern english IMHO what 2 thess 2:3-4 says is :
2Th 2:3 Let no man deceive you by any means: for that day shall not come, unless the departure ((the rapture)) comes first, then the man of sin will be revealed, the son of perdition;

2Th 2:4 Who opposed and exalted himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, showing himself that he is God.

Lewis  Brackett