Suppose you can see the future. Suppose you foresee that tomorrow I will die in a terrible automobile accident.
You warn me of my impending death. I heed your warning.
However, when tomorrow comes, for whatever reason, I decide to ignore your warning. I get in my car, go for a drive, and then die in a terrible accident just as you predicted.
Yesterday, you and I both thought that I was saved from this accident. One might even say that we both had faith that I was saved. But, in the end, it turns out that I was not, in fact, saved from death. It turns out, then, that our earlier faith was a false faith.
Yesterday, you and I both sincerely believed that I was saved. Today, it turns out that we were both wrong. Can it be said that I was ever truly saved from this accident if, in the end, I was not saved from the accident? No. Not if the word salvation actually means anything, that is.
As believers, we say that we are saved. The thing that we are saved from is the wrath of God to come (Romans 5:9; 1 Thessalonians 1:10). The wrath of God that will be poured out on unbelievers on judgment day (Romans 2:5; Ephesians 5:6; Colossians 3:6). If, on that day, we are not actually saved from that wrath, then we cannot be said to have ever been saved in any meaningful sense of the word.
The only way you and I, as believers, can have any assurance of salvation—the only way we can truly be said to be saved in any meaningful sense of the word—is if God, the Righteous Judge himself, assures us somehow that, on judgment day, we will, in fact, be saved.
That is exactly what he has done. He justifies—declares righteous—his people, and he then promises that all who are justified will, in the end, be glorified (Romans 3:24ff.; 5:1; 8:30, 33).
God's Word teaches that salvation consists of an unbreakable chain
starting with the foreordination of his people and terminating with their glorification—their being made like Christ (Romans 8:28-30). God ensures that this chain remains unbroken by working in us (Philippians 2:13; Hebrews 13:21; cf. 2 Corinthians 4:12; Ephesians 1:19; 2:10) and for us (Romans 5:6; Ephesians 1:11; 2 Timothy 1:9). He recreates us (John 3:3ff.; 2 Corinthians 5:17; Titus 3:5; 1 Peter 1:3, 23). He writes his law on our hearts (Hebrews 8:10; 10:16; Jeremiah 31:33). Christ intercedes on our behalf (Romans 8:34; Hebrews 7:25) as does his Spirit (Romans 8:26-27). The Father gives us to the Son (John 6:37). The Son does not cast us out, rather, he raises us up (John 6:38-40). Nothing can wrench us from his grasp or separate us from his love (John 10:28-29; Romans 8:35ff.). Our salvation, then, is ensured by God's work for us rather than our work for him (Romans 9:16). This means that, if a believer loses his salvation, it is not because he has failed God but because God has failed him. This is an obvious impossibility.
On judgment day, there will be only two groups—those whom God foreknew (Romans 8:29; 1 Peter 1:2), and those whom he never knew (Matthew 7:21-23).
There will be none whom he once knew.