“Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name of the only Son of God.”
“The plain sense of this text is that if one believes now, he is not condemned (lost) now and will not be condemned later (cf. Rom. 8:1).” 1
If the interpretation was left simply as, ‘the plain sense of this text is that if one believes now, he is not condemned (lost) now’, there would not be much to dispute. But developing the OSAS interpretation that bit further by adding ‘and will not be condemned later (cf. Rom. 8:1)’ is unwarranted, and the reasons are fourfold:
1. The text does not say it. The text does not say, ‘Whoever once believed in him is assured of never being condemned, no matter what.’ It says, “Whoever believes in him is not condemned…” Whoever believes (presently) is not condemned (presently). But that raises the question – why aren’t those who believe condemned? The answer is: because through the sacrifice of Christ, their sins have been forgiven. But that raises another question – what sins are forgiven? Past sins? Present sins? Future sins? The Scriptures are clear: only past sins that have been repented of are forgiven (cf. Luke 13:3, 5; 17:3-4; Acts 2:38; 3:19; Rom. 3:21-25; 2 Pet. 1:9; 1 John 1:9; 2:1-2). The promise of not being condemned is conditioned upon belief, and is applicable only for past sins, not present ones that haven’t been confessed, nor future ones. For a powerful refutation of the idea that future sins are already forgiven, see Dr. Michael L. Brown’s Hyper-Grace (2014: Charisma House).
2. The context does not demand it. There is nothing in the context of the passage that demands that it be interpreted to mean that one act of faith unconditionally assures someone of never being able to resume a state of unbelief (condemnation). In fact, other Scriptures (such as Rom. 6:23; 1 Cor. 11:32; Jas 5:12) indicate that there is a possibility of believers falling under condemnation, via sin.
3. The logical conclusion of such a belief is the same lie that Satan told Eve: that even if you sin, ‘you will not surely die.’ The OSAS interpretation was that “if one believes now, he will not be condemned later.” On the contrary, the Scriptures say that the wages of sin is death (Rom. 6:23), and what’s more, that was written to believers. Even for the believer, the wages of sin is still death. It would appear that the apostle Paul didn’t believe OSAS.
4. Citing Romans 8:1 as proof of OSAS is circular reasoning. Romans 8:1 says:
“There is therefore now no condemnation for those who are in Christ Jesus.”
Note that there is now no condemnation for those who are in Christ Jesus. For this passage to have any force, it must first be assumed that ‘once in Christ Jesus, always in Christ Jesus’, which is the very point in question. In other words, we would have to assume OSAS in order to prove OSAS. See also Feedback: A Christian Can’t Lose Their Salvation Because They’re a New Creation
John 3:18 does not prove unconditional eternal security. As with the previous proof-text, John 3:15, the promise of security is only for those who believe (i.e., those who believe now, in the present). There is no promise of security for those who once believed. Indeed, we are warned in God’s Word that if a righteous person turns away from his righteousness, he will be destroyed. His past righteousness will not be taken into account (Ezekiel 18:24). One act of faith sometime in the past is not enough to guarantee an entrance into the eternal kingdom of our God.
At the very best, John 3:18 is inconclusive.
1 Geisler, Systematic Theology, p. 306