Justification by God's Word of Promise Alone
"One thing," Martin Luther writes in the Freedom of a Christian (1520), "and only one thing, is necessary for Christian life, righteousness, and freedom. That one thing is _______." Knowing Luther to be the author, we're quick to assume that "faith" belongs in the blank. And not without reason. Luther is keen to emphasize in this work and others the instrumental role that faith plays in laying hold of Christ and his perfect righteousness as the basis of our own perfect standing before God. But that's not where Luther starts. "That one thing," he writes, "is the most holy Word of God, the gospel of Christ."
Luther's doctrine of justification by faith alone is prefaced and contextualized by a doctrine of justification by God's Word alone; or more precisely, a doctrine of justification by God's word of promise alone. It's well worth tracing his own train of thought on this score, because it helps us understand why faith, in Luther's (and hence Protestant) thought, ultimately plays the pivotal role that it does in apprehending salvation.
"The soul [that] has the Word of God," Luther begins, "is rich and lacks nothing since [that Word] is the Word of life, truth, light, peace, righteousness, salvation, joy, liberty, wisdom, power, grace, glory, and of every incalculable blessing." Yet even this requires qualification since God's Word, in Luther's estimation, "is divided into two parts," and it's properly the latter "part" that proffers the benefits just named. God's word consists of "commandments and promises." The former "show us what we ought to do but do not give us the power to do it. They are intended to teach man to know himself, that through them he may recognize his inability to do good and may despair of his own ability." Once man has despaired, then God addresses him with His word of promise, that word that properly justifies. "Here the second part of Scripture comes to our aid, namely the promises of God." God's word of promise is the word "concerning his Son, who was made flesh, suffered, rose from the dead, and was glorified" on behalf of sinners in need of rescue. This, again, is properly the word that justifies: "To preach Christ means to feed the soul, make it righteous, set it free, and save it."
And thus we arrive at faith. For faith, and only faith, is the appropriate response to this word of promise. "Faith alone is the saving and efficacious use of the [promissory] Word of God.... Therefore it is clear that, as the soul needs only the [promissory] Word of God for its life and righteousness, so it is justified by faith alone and not any works."
As Luther goes on to explain, good works -- that is, any human striving after righteousness -- are really a blasphemous response to God's word of promise (if done in the hopes of securing salvation). After all, it is the height of ingratitude and unbelief (not to mention futility, given our sinful condition) to try to earn that which one offers to us as free gift. By way of analogy, the appropriate response to an invitation to my family's house for dinner is not to show up on our doorstep with a crock-pot and accoutrements in hand, but just to show up, hungry and confident that you will be fed. Efforts to merit that which God freely gives voice refusal to believe that God is as generous and liberal as he declares himself to be when he bids us to "come [and] buy wine and milk without money and without price" (Isa. 55.1).
Luther's doctrine of justification by God's word of promise alone provides critical context to his doctrine of justification by faith alone. It reminds us that there is in fact a (divine) logic to the instrumental role that faith plays in appropriating Christ and his righteousness as the ground of God's judicial declaration of our perfect standing before Him. Too often, I think, our Protestant talk about justification by faith alone fails to reflect that logic. Too often, that is, we fail to meaningfully consider the (promissory) nature of the divine word that faith answers to, and spring too quickly to a discussion of faith vis-a-vis love, hope, works, etc. We thus stand in danger of treating faith as some arbitrary thing that God has seized upon, a hoop to jump through (as it were) before he grants us entrance to eternal joy in his presence -- as if, indeed, he might have chosen some other thing (whether love, hope, or a daily diet of cheeseburgers).
Luther's doctrine of justification by God's word of promise alone also helpfully reminds why a doctrine of justification by faith plus works (however conceived) is so heinous. A doctrine of justification by faith plus works is not merely dangerous to our souls (though it is that). A doctrine of justification by faith plus works twists God's word of free promise into a word of conditional promise, a word that dangles life in front of us if we will only meet some demand. As such, a doctrine of justification by faith plus works constitutes a perverse theological claim, representing God as someone or something different than he reveals himself to be in his own accomplishment of our salvation and application of the same to us.
Proper acknowledgement of the promissory nature of God's justifying word to us, by way of contrast, helps us appreciate why exactly "true faith in Christ is a treasure beyond comparison," a treasure "which brings with it complete salvation."