For the most part it describes Jews who denied Christ and returned to Judaism. But it applies equally to anyone else of that time who fell away from Christianity and returned to whatever it was they were doing before - Emperor worship, Mithraism, etc. Part of that is denying Christ either explicitly or tacitly. The Romans were good at forcing people to deny Christ and worship images of the Emperor;
Rev 3:8 “I know your works. See, I have set before you an open door, and no one can shut it; for you have a little strength, have kept My word, and have not denied My name.
My thought is that an "apostate" is a so-called Christian who turns against doctrines that are time-tested, biblical, and accepted generally in the Church as Christian doctrine, fundamental to the faith, or part of the creeds. When a Christian turns to immorality, or to ungodly behavior such as cheating, theft, etc., then that is simply a moral lapse, and not necessarily teaching that these things are okay. But if a person *teaches* that his unChristian behavior is biblically acceptable, then he is an apostate.