"The Spirit expressly says that in later times some will depart from the faith by giving heed to deceitful spirits and teachings of demons" (1 Timothy 4:1)
Here we have another clear example from Scripture that people who are true Christians will turn from the faith. The word translated as "depart" is the Greek word "aphistemi" which is the verb form of the noun "apostasia" from which we derive the English word "apostasy."
Paul ends this particular instruction to Timothy by telling him, "Take heed to yourself and to your teaching; hold to that, for by so doing you will save both yourself and your hearers" (4:16). Therefore, we can be totally sure that Paul certainly has the issue of salvation in view. In order to depart from the faith, one must first be in the faith and therefore we can see clearly that Paul is talking about people who were Christians who then turn to the teachings of demons. It is for this reason Paul exhorts Timothy to hold fast the teaching delivered to him and to teach it to others so that he and others will not be deceived by these false teachings and turn from the faith to the doctrines of demons.
At 2 Corinthians 13:5, Paul exhorts the Corinthians to test themselves to see if they are really in the faith. And John tells us to test the spirits to see if they are from God. These ideas all deliver the same general message. One must be on guard for false prophets because false teachings can and do lure one from the faith into something which is not the faith at all. Hence, there can be no doubt that Paul here is discussing Christians who will fall from the faith and jeopardize their salvation because they have been deceived by false teachings.