“There is a long-standing division within Christ’s church as to whether salvation, once acquired, can ever be lost. Some believe in an eternal security “once saved, always saved” (OSAS) doctrine. I once believed that but I think the overwhelming weight of the New Testament speaks against it. Persistent and unrepentant sin of an egregious sort, I believe, can get one excluded from eternal life. There are too many texts that make the point clear: for example (and I am making no attempt at being exhaustive),
John 15:2, 6; Rom 8:12-14; 11:20-22; 1 Cor 3:17; 6:9-10 with ch. 5; 9:24-10:13; 15:1; 2 Cor 6:1; Gal 3:1-5 with 5:2-4; 5:19-21; 6:7-9; 1 Thess 4:3-8; Col 1:23; Eph 4:17-19; 5:3-6; 1 Tim 3:6; 4:1; Heb 2:1-4; 3:7-4:13; 6:4-6; 10:26-29; 12:15-17; 2 Pet 2:20-22; 3:17; Rev 2:5; 3:3-5; 3:16; 22:19; Matt 5:13, 29-30; 6:15; 18:23-35; 22:11-13; 25:14-30 (= Luke 19:11-27); Mark 4:16-19; 13:13, 20-22, 32- 37; Luke 13:6-9; 14:28-33.
Developing the argument for this would require another paper at another time. Suffice it to say, no one can know for certain when a believer crosses the line into falling away. Not even in the case of the incestuous man could Paul make that call; Paul simply referred to him ambiguously as “someone who goes by the name of brother” (1 Cor 5:11). But he could warn the offender, as he frequently warned all his followers, that an immoral life put one at high risk of not inheriting God’s kingdom. By way of analogy, a parent can’t say for certain, if his or her child skates out into thin ice, precisely when (or even if) the child will fall through the ice. Nevertheless, the parent can warn the child of the grave danger involved in traveling onto the thin ice. It is not a question of earning salvation (which the New Testament authors clearly state cannot be done) but rather of letting Christ live within oneself, to which faith (if it is true faith) always says “yes.”
The oft-cited Rom 8:35-39 listing all the things that “will not separate us from the love of Christ” or “the love of God in Christ” speaks only of things external to ourselves: persecution, a deprivation of material goods, angels and other spiritual powers, death. The remark “nor any other creation [or: created thing]” (8:39) appears to refer primarily to the material structures of non- human creation or at least created things external to one’s own self (compare 8:18-23, which distinguishes “creation” from the sons or children of God). The lists do not include “a life lived under the control of sin operating in human members” and for good reason: Paul has already stated clearly that such a life leads to death (6:16, 21; 8:12-14).”
– Robert A. J. Gagnon, link