Faith Upholds The Whole Law
Our default mode of operation is law. We're born under the law. Natural born legalists. Moralism pumps through our veins like antibodies. Fairness and justice are words we learn long after we've put them into practice as a relational standard. This is how we function. We go along in this way, keeping the law, inventing new laws, adding by-laws to old laws, all so we can establish a status quo under the law. At least, that's what we hope. We establish the laws, obey them, and work within their boundaries out of fear of punishment or love of reward. That's why we cling to the law even after it frustrates and exasperates us. We're eternal optimists just hoping for one more big score. One more chance to be right, righteous, to have the scales of justice tilt toward the other guy for once.
The only reason we do anything in this life is out of fear of punishment and love of reward. And as much as the idea of free will titillates us, we don't do anything spontaneously or freely because fear and love of the law is always driving us forward. We think, speak, and act based on our aversion to punishment and force. "No need to call the police about this, we can work it out," we say, unless we're the wronged party then, by all means, "Someone call the police!"
But, then God speaks His Word of Law to us and we'd rather the Law not exist. In the depths of our heart, even for all our talk of loving the law for what it offers us, we secretly hate the law when it's God speaking to us. Once God speaks His Word of Law to us, we become enemies of the Law. More than that, when God speaks His Word of Law to us, the Law increases sin, as St. Paul says in Romans 5. Why? Because the more God demands from us in His speaking the Law, the more and more we hate God because He demands we do the impossible.
If God made demands that we could actually do with our hands, pick up and use like a woodworker uses a saw or a mechanic a wrench, we could follow through and accomplish what God demands. But, the Law of God, unlike all human laws, is spiritual, as St. Paul says in Romans 7. What that means then, is that nobody can accomplish what God demands unless it bursts freely and spontaneously from our hearts. But none of us can give our hearts to God unless His Spirit is already at work in us. Then we do love the Law for its own sake, because we recognize it's God our heavenly Father who's speaking to us for our good. But if God's Spirit isn't at work in us then the Law increases sin, we grow to hate God, and we call the Law "bad" even though it's holy and good.
So, as Martin Luther said:
"...it's one thing to do the works of the law and quite another to fulfill it."
We live every day under laws we've invented. All of our thinking, speaking, and doing is a work of the law. However, that does not mean we do it freely, out of love for the law. No, instead we do every work of the law in fear of punishment and love of reward. Likewise, when God says, "Do this and you will live," we go about under the illusion that we have the ability to accomplish what God demands of us. In fact, we don't. We're enemies of God and hate God in our hearts for demanding the impossible. We're at a loss to do what God demands. We're useless unless the Holy Spirit works in us to freely and spontaneously do what God demands in His Word of Law.
This is why, as Martin Luther writes:
"faith alone makes someone just and fulfills the law; faith it is that brings the Holy Spirit through the merits of Christ. The Spirit, in turn, renders the heart glad and free, as the law demands. Then good works proceed from faith itself. That is what Paul means in [Romans] chapter 3 when, after he has thrown out the works of the law, he sounds as though he wants to abolish the law by faith. No, he says, we uphold the law through faith, i.e. we fulfill it through faith" (Martin Luther, Preface to Romans).
God's Spirit is given to us only through faith in Jesus Christ, as Paul says. And faith comes to us through God's Word of Gospel, that gives us Christ for our salvation. With the giving of Christ through His Gospel Word, God reveals to us that all the Law's demands are hung on Jesus for us, accomplished by His obedience for us, and fulfilled by His faithfulness to His promises and saves us from sin, death, and Satan. All these things we receive through the faith worked in us by God's faithfulness. We don't have to fear punishment or hope for a well-earned reward, because Christ received our intended punishment upon Himself. He earned the reward of forgiveness, life, and eternal salvation for us on the cross.
Faith in Christ Jesus is the fulfillment of the Law. All from Him to us is gift through faith, even faith that, it turns out, upholds the whole Law.