Well, I suppose this question boils down to how I would distinguish myself from the next guy who might have the same knowledge I do and says the same things I say, but then he falls away permanently. I do not think it is possible to "prove" that difference to you or anyone else. Only God and I know what my faith truly is. So, I can report to you things like I know that God has actively worked in my life since my conversion, and that those workings directly match what is foretold in the Bible for all Christians, but you can't "know" it the way I "know" it. The same is true in reverse.
We can only know for sure what is in our own hearts. I personally have never known someone who "convinced" me he was a Christian and then later fell away permanently. That is not at all to say that very convincing people have never fallen away, of course that has happened, but I can't compare what the heart of a false believer looks like (from the outside) to my own through my experience.
From a practical standpoint, how does it benefit me as a Christian to disregard how the assurance and perseverance verses speak personally to me? One possibility would be if they "moved" me to sloth, but they do not because I am well aware of the multitudes of other verses that cover the subject. So, if my "objective" and honest reading of those verses is the Reformed view, then why should I reject that? They bolster my faith and thanksgiving to God.
Now, if it turns out that I really am a pretender, then by my own beliefs there is still nothing I could ever do to change that. God would have been using me in my pretender role all along for His purposes. Glory be to God for that if it was true. This is partly why I can have assurance. WHATEVER the truth is, it's all in God's hands, not mine. Therefore, I will spend my life living with assurance in Christ and His promises, as I understand them, and not worry about whether I am a pretender. Either way, it will be to God's glory, and that is all I could ever truly want.
If you knew you were elect, you would never acknowledge the possibility that it could turn out that you are really a pretender. Therefore, since you don't know whether you are elect, you don't know now that on Judgment Day you will be welcomed into His Kingdom.
-A8
It's true that we cannot know what is really in someone's heart... unless God explicitly tells us, as in the case of Simon the sorceror in Acts 8. That chapter tells us "Simon himself believed and was baptized." Not that he claimed to believe, or seemed to believe, but that he DID believe & was baptized. Yet according to Peter, Simon is subsequently in a condemned state when he addresses him in verses 20-23.