As simply as I can say it, the new birth is irreducibly relational; you are born again when you trust Jesus Christ to save you. There is no consistent reading even of John’s gospel, let alone the whole New Testament, that successfully presents a single proposition as the content of saving faith. The thing can be described in propositions to an extent, but it’s not actually a matter of subscribing to propositions. Propositions didn’t die for your sins; Jesus did.
Many people balk. “How does one have assurance?” they want to know. “What must I believe, to be sure that I am saved?”
Ah, my friend, if you’re thinking in terms of “what I believe,” you’re missing the point: it’s not “what,” but Who! It isn’t about “correct belief” or “fulfill[ing] the ‘belief’ condition.” The news is far better, and far simpler, than that.
This Jesus that we meet in (say) the pages of John’s gospel — He wants to save you, sacrificed everything to save you, and He means to see it done. You need not fret about fulfilling conditions or fussing about with propositions any more than you need fret about your insufficient moral merits. Rest assured, you are inadequate! Whether we’re talking about your morals or your theology, you are inadequate! The whole point is that Jesus met the conditions for you, and He will save you. He’s got you; your assurance comes from knowing that it’s Him that’s got you.
Theologically speaking, that’s sufficient. Practically, there’s another avenue as well. Eternal life just is knowing God (Jn. 17:3) and it’s not something you hope to get eventually, it’s something you have now (Jn. 5:24). Assurance naturally grows in the living of it. I have the paperwork to prove that Kimberly married me, but where do I get my day-by-day comfort and assurance that our relationship is what I think it is? Not from looking at the paperwork – what kind of relationship would that be? I am assured that I know Kimberly in the day-to-day living with her, and so it is here, because like a good marriage, eternal life is not having your papers in order; it is knowing a Person.