Tangent or obvious question then follows:
Since A MULTITUDE of believers today are "certain" they are saved, and they are not, what then?
They have "assurance of salvation", from themselves and others, yet are told to get lost on Judgment Day, by the Judge, the Perfect, Righteous Judge, our Savior, the Messiah Jesus.
I'd like to add a bit to the excellent work of
@Anthony D'Arienzo on this thread.
Romans 8:1.
'There is therefore now no condemnation to those who are in Christ Jesus........' That seems remarkably clear to me, but the question is asked, how do I know if I am in Christ Jesus? Obviously, as you say, there are multitudes today who believe they are saved, but are not.
'........Who do not walk according to the flesh, but according to the Spirit.' The mark of the Christian is that he has a new principle of righteousness within him, which is the Holy Spirit. Every Christian has the Holy Spirit.
'That which is born of the flesh is flesh, and that which is born of the Spirit is Spirit' John 3:6).
'So then, those who are in the flesh cannot please God. But you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. Now if anyone does not have the Spirit of Christ, he is not his' (Romans 8:8-9).
But the Scripture tells us that in every Christian there remains a relic of the old sinful nature ('flesh'). Paul is writing to Christians when he says,
'Therefore do not let sin reign in your mortal body, that you should obey its lust' (Romans 6:12). And to the Christians at Colosse, he writes,
'Therefore put to death [or
'mortify']
your members that are on the earth' (Colossians 3:5). Not that he doesn't say, 'put yourselves to death,' for the very good reason that it's already happened (v.3).
So my answer to the question, how do I know if I'm saved, is this: have you seen yourself as a lost sinner, justly under the condemnation of God? Have you repented of your sin and trusted in
nothing else but the blood of the Lord Jesus, shed for sinners on the cross? And now are you seeking, by the Spirit, on a daily basis to put to death that relic of sin that dwells within, not you, but your flesh (c.f. Galatians 5:16-18)? Does it grieve you when you fail to do this as thoroughly you should, and do you confess your failure to God (1 John 1:8 - 2:2)? If you can answer all these questions in the affirmative, I believe that is a powerful indication that you are born of the Spirit and a child of God. And if you are, then Romans 8:1 applies to you.
I strongly recommend reading
The Mortification of Sin by the Puritan John Owen. It is not an easy read - Owen seldom uses one word where ten will do - but it will reward the effort. Alternatively, a chap named Kris Lundgaard has written a book based on Owen's work, but much easier to read, called
The Enemy Within (P & R Publishing, 1998).