According to Scripture, people who profess to know Christ at one time but later deny Him were never really saved to begin with. First John 2:19 says, "They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, in order that it might be shown that they all are not of us." A true believer will never depart from the faith (Philippians 1:6), so those who do so are revealing that they were never truly saved (John 8:31; Hebrews 3:14).

Even true Christians can sin, however, and because of that may lack assurance of salvation (Psalm 51:12). A failure to grow spiritually can also rob us of the confidence that we are God's children (2 Peter 1:9). But anyone indwelt by the Holy Spirit is secure eternally, because He is the "deposit guaranteeing our inheritance" (Ephesians 1:14).