It is a falling away, a withdrawal, a defection. Used twice in the New Testament, in the Greek original, to express a total abandonment of the faith.
This was a warning that was foretold to the believers. Question: If apostasy is something that only lost people can do, as many of our denominational teaching espouse, than why was it important that the warning was given to believers and to the church?
The first warning from 1 Tim.4:1-3: Questions: In verse 1 it says that there are those who will depart from the faith. How can you cannot fall away from, defect from, or withdraw from something you have not been in? Secondly, lost people are already the children of Satan, deceived and speak the teachings of the enemy, so how can lost people leave the faith that they were never in, and devote themselves into something they have always been in?
1 Timothy 4:1-3 ESV Now the Spirit expressly says that in later times some will depart from the faith by devoting themselves to deceitful spirits and teachings of demons, (2) through the insincerity of liars whose consciences are seared, (3) who forbid marriage and require abstinence from foods that God created to be received with thanksgiving by those who believe and know the truth.