What's His Is Ours
But contrary to popular Protestant polemics, nobody in the whole Christian tradition—certainly not Thomas Aquinas or his diverse cohort of fellow medieval scholastics—has ever seriously disputed that justification is by grace. The problem is that grace, however mighty a concept, involves a certain ambiguity. What exactly does divine grace do? Luther believed that some of his predecessors, chiefly the German scholastic Gabriel Biel, held that grace is divine assistance to a human nature eminently capable of pursuing righteousness, but too lazy or disinclined to achieve it without some prodding. As far as Biel was concerned, it was already amazing grace that God had provided any opportunity for salvation after the Fall; we should be grateful, then, that God gives us the chance to do our very best in amassing good works. In short, grace would enable us, with God's help, to save ourselves. But saving ourselves would remain our obligation. And to Luther, that was not good news. That was the marriage vow taken hypothetically and conditionally, waiting to see how the spouse would shape up in the end—and destroying the spouse in the process.
So for Luther, explaining why justification is by faith starts with giving a clearer account of grace. Our faith begins with the Holy Spirit graciously coming to us, seeking us out and tracking us down. The Spirit's work is to bring the crucifixion and resurrection of Jesus Christ to bear on our own biographies and circumstances, beyond all limitations of time and space. Through baptism and preaching, the Spirit says to you, to me, to everybody: "While we were still sinners, Christ died for us" (Rom. 5:8), and so the Father promises to forgive us all our sins, grant us Christ's own righteousness, and raise us up to new life in the heavenly kingdom.
That is the promise that the Spirit brings. But we can't see these realities now. God indulges in empirical proofs on very rare occasions, and even those don't always persuade—think of Gideon and the wool (Judges 6:36-40), or the disciples at the end of Matthew's gospel who see the risen Jesus yet still don't believe. The one and only way to receive a promise is to believe in it. Believing in God's promise counts as righteousness (Rom. 4:5).
But still the anxiety lingers: How do I know that I really have faith in the promise? True faith, enough faith—justifying faith? If the answer lies with me, then I'm lost, as much as when I depended on my good works. Thankfully, the answer lies with God. For it is Christ, brought from heaven to earth by the Spirit, who does the believing in and for me, much as Paul says: "I no longer live, but Christ lives in me" (Gal. 2:20). Christ dwells in me, and so the Father judges me as righteous—not as a legal fiction, but as a truth, because the Father has truly given me Christ's own righteousness, and through the Spirit I have received it. In a right understanding of justification, there is no competition between "the faith in Christ" and "the faith of Christ." They are the same reality.
That's why Luther, following Paul in Romans 3:28 and elsewhere, argued that we are justified, made right with God, through our faith. And this faith is so great that it can even give us assurance of our salvation, a position that horrified his opponent Cajetan, the Italian cardinal, who could only hear it as a presumption on God, and even as "build[ing] a new church."