It is a falling away, a withdrawal, a defection. Used twice in the New Testament, in the Greek original, to express abandonment of the faith.
I just finished reading R C Sproul's definition of apostasy and according to him only lost people can apostatize. His argument was emotional at best using ten dollar words that said nothing. When we are emotionally and mentally bound to a certain doctrine, we need to find intellectual and emotional answers for Scriptures that doesn't fit our doctrine. I call those Scriptures our "Embarrassing Scriptures." These are the Scriptures that blow holes in our man-made intellectual belief system, yet we either ignore them or use some lofty explanation to dummy them down to fit our doctrine.
So, to those who believe that a lost person commits apostasy here is my question. Exactly what is a lost person falling from, revolting against, defecting from? A man is never as bad as he can be, for no matter what immoral word, deed or thought he commits, he can outdo himself and be worst. However, even though a man is never as "BAD" as he can be, when he is spiritually lost, he is as "BAD OFF" as he can be. There are no degrees of lostness. A man can't fall deeper into being lost. A lost man can't defect to another level of being lost.
Our Lord Jesus Christ spoke about a great apostasy from Christianity in the last days in the Book of Matthew. Remember that He is warning His followers. If this message was for lost people why didn't our Lord find a group of heathen or religious lost leaders and deliver it to them?
Matthew 24:9-13 ESV “Then they will deliver you up to tribulation and put you to death, and you will be hated by all nations for my name's sake. (10) And then many will fall away and betray one another and hate one another. (11) And many false prophets will arise and lead many astray. (12) And because lawlessness will be increased, the love of many will grow cold. (13) But the one who endures to the end will be saved.
Let's look at what our Lord said without our preconceived denominational biases. In verse 9 who is being delivered up for tribulation to be put to death? The believers. In verse 10 it will be the believers who will fall away. In verse 11 the lost false prophets will lead believers astray. It certainly can't be lost religious people leading lost people astray, they are already deceived. In verse 12 the love of many will grow cold. Only believers know the love of God. What love do lost people have? Phileo, Eros, and Epithumia, all of which are emotional and conditional. In verse 13 again we see that our Lord is speaking about saved people. What are they enduring? The tribulation of verse 9. The watching of many falling away and betraying one another of verse 10. The temptation to follow false prophets and becoming deceived of verse 11. The watching coldness of fellow believers of verse 12.
The Apostle Paul piggybacked on Matthew 24, in his warning to the church about the coming tribulation.
2 Thessalonians 2:1-3 ESV Now concerning the coming of our Lord Jesus Christ and our being gathered together to him, we ask you, brothers, (2) not to be quickly shaken in mind or alarmed, either by a spirit or a spoken word, or a letter seeming to be from us, to the effect that the day of the Lord has come. (3) Let no one deceive you in any way. For that day will not come, unless the rebellion comes first, and the man of lawlessness is revealed, the son of destruction,