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 am not speaking about those who were lost and thought that they were saved. That is covered in Matthew chapter 7.
Matthew 7:21-23 ESV “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. (22) On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ (23) And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’
In Matt.7:21-23 this is lost religious people trying to convince the Lord on the day of judgment of all the great things they did. In verse 21 they confessed the right name, "Lord, Lord," but our Lord Jesus Christ makes it clear that is not enough.
In Matt.7:22 these lost religious people start running down their list of all the wonderful things they did in the name of our Lord Jesus Christ. They prophesied, they cast out demons, they did mighty works. Notice that the men were not making a statement of fact, but was asking a question, "did we not"
In Matt.7:23 Our Lord answering the question in a way that is surprising. Our Lord said that He NEVER knew these men. They were religious and religiously active, but NEVER had a relationship with the Lord Jesus Christ. He then told then to depart away from His presence. The next statement is amazing, He called them "workers of lawlessness." Our Lord Jesus Christ was basically saying that they used His name but had no right to. And He never acknowledged any credibility to their claims of great and mighty works such as prophecy, casting out demons, and mighty works. In fact He had a name for that which we might look at as great things within our church. Because they did not know Him, their religious works were explained as works of lawlessness.
I have known many religious lost people in my ministry. Those who got baptized and joined the church and basically treated the church like their social hour each week.
Now that we have covered the fact that there are lost people who think they are saved, look like they are saved, smell like they are saved, talk like they are saved, and have others deceived that they are saved, we can move on. I am not addressing these people. These are those who have NEVER met the Lord. They can't fall away from the faith because they have never been in the faith. You can't defect, revolt and fall away from something you have never owned or embraced. Embracing a religious institution or denomination is not being "in the faith." I am addressing the actual falling away of those who are truly born again.