Why is it that faith saves? What's so special about faith that it brings such benefits?
Because here's how the whole deal is usually set up:
First we insist that God does not save us by our works. No sir, we believe in 'justification by faith alone.' Therefore it's not that God is armed with a clipboard and some binoculars waiting for an external moral act in order to flick the 'justification' switch. How ridiculous. No, no. Instead we imagine God (with clipboard and brain scanner) eagerly seeking for a certain mental act within us. And then He'll zap righteousness into our account.
Yeah. That's much more reformed...
But honestly, for many, that is the doctrine of justification by faith alone in a nut-shell.
Yet for the thoughtful who've been reared on such teaching it raises big questions. Like, why faith? Is it just that 'faith' keeps us humble and God simply wants to remind everyone who's Boss? In which case why give us Christ's righteousness at all? Why not just leave us in a sort of righteousness limbo forever - that'd keep us humble right? And what's the link between this act of mental assent and that imputation of saving stuff?? It all seems so arbitrary.
And it would be completely arbitrary so long as we keep Christ out of the discussion. But once Jesus is central - and by that I mean the Person of Jesus (not just the Provider of a Perfect Righteousness) - then things start to fall into place.
Because faith is receiving Jesus Himself (John 1:10-12). He gives Himself to the world in life and death, He pledges Himself to us (marriage style) in the gospel. When we hear the gospel rightly we are swept off our feet by such a proposal and find ourselves saying "Yes." That is faith. And by faith we are united to Christ. In that union we have our salvation because salvation is all in Jesus.
So there's nothing at all arbitrary about the connection between faith and salvation. Because there's nothing arbitrary about the link between a marriage vow and marriage union. Once we are united to Christ by faith, then of course we instantly have His name, His wealth, His family connections. Of course then instantly we have the righteousness of Christ imputed. But it's not an impersonal imputation in response to an impersonal faith!
Justification by faith does not mean "being zapped because of mental assent." But we'll never get that unless we put union with Christ at the centre.