"But the Spirit explicitly says that in later times some will fall away from the faith, paying attention to deceitful spirits and doctrines of demons, by means of the hypocrisy of liars seared in their own conscience as with a branding iron, men who forbid marriage and advocate abstaining from foods, which God has created to be gratefully shared in by those who believe and know the truth."In this passage, Paul describes an apostate as one who departs from the faith. Some individuals have erroneously interpreted Paul's statement here to mean that these individuals were previously saved. However, this is a reference to a departure from the gospel itself, not from personal saving faith. We must not confuse "the faith" with personal faith in Christ Jesus. These are two distinct concepts (cf. Jude 3). "The faith" refers to the body of truth (inclusive of the gospel) that defines Christianity. "Personal faith" is submission to the gospel, which is a portion of that truth (cf. 1 Pet. 1:22; 2:8; 3:1; 4:17). Of course, in order for someone to depart from the faith, they must have professed to have believed that faith. Otherwise, how else could it be said that they depart from it? In other words, an apostate is one who professes to believe the truth for a time, but later turns from that profession. It is crucial for you to understand that outward profession may or may not be the product of genuine saving faith. There is no way of telling from a profession if the faith professed is genuine. True faith can only be seen through a transformed life consistent with belief in the truth. The fact that Paul indicates it was from "the" faith that they departed, rather than from "faith," proves beyond a shadow of a doubt that this passage is not suggesting that they were previously saved.
Borrowed and revised from Sam A. Smith's The Biblical Doctrine of Apostasy.