Take for instance the case of your neighbor or a friend that you know is not a believer in Christ. What is the difference between you as a believer and the other? The constitutionalist would say that there is a constitutional or ontological difference: the unbeliever’s spirit is dead, or the unbeliever’s nature is totally wicked (though he may sometimes seem to be doing good), while your spirit has been made alive or recreated and you have been imparted a divine nature. Thus, there is a constitutional difference between you and the unbeliever.
The non-constitutionalist, on the other hand, will say that there is no constitutional difference between the two of you; the only difference is faith, which in the believer is present, but in the unbeliever is absent. Christ and the Spirit indwell us by faith (Eph.3:17; 2:22). Faith is epistemic condition, involving free choice; it is not an ontic property related to nature and being. Now, the presence of doubts at times doesn’t make one an unbeliever, because doubt is not committed unbelief. A person might observe some doubts arise in his mind at times, however, they are soon quelled and put to silence as he seeks the Lord. One good example of this is the situation of Asaph in Psalm 73, and the situation of Job in the book of Job. Similarly, one who has not heard the Gospel message clearly has not been given the clear-cut chance to choose unbelief. However, those who know and choose not to believe suffer condemnation (Jn.3:16-20; Jude 1:5).
Thus, a person who does not have the faith of Christ may fall into, at least, one of these three positions: ignorance, or doubt, or unbelief. The Bible says that God overlooks sins during the times of ignorance (Acts 17:30); however, that does not excuse people from judgment according to conscience (Rom.1:19-32; 2:12-16). The situation of doubt is a temporary time of suspension and not of decision; it is volatile and can lead to either belief or unbelief. However, those who choose unbelief forfeit the salvation of God.
Is it possible for someone who has accepted the Gospel to forfeit faith and salvation? Yes, it is. People can drift away from faith (Heb.2:1), make shipwreck of their faith (1Tim.1:19), and can fall away from faith (2Pet.3:17; Heb.6:6; 2Thess.2:3).