This is a big point that Oswald Bayer makes in his *Theology the Lutheran Way* - that the Reformation was essentially a turning away from an attempt to harmonize philosophy and theology, and recognition that theology lives in a different world than ideas. In fact, Bayer can say: "We are called to fight against the attempt to turn the gospel into a theory." And doesn't matter much a theory of WHAT. But if not theory, then what?
The Reformers turned to the category of language, of speech, and there discovered a different approach. The Gospel is and remains essentially communication. God is speaking, addressing sinful human beings. And it is HOW He speaks and WHAT He speaks that marks this communication as Gospel. That is, He speaks PROMISE. Not the "if you do this, then I'll do that" kind of promise (which we might call a deal, but not strictly a promise), but a Word that simply announces what He HAS done, IS doing, and WILL do for you in His Son.
When Lutherans are concerned about the mixing of law and Gospel, they are concerned with the loss of the promise-nature of the Gospel - and it is the promise nature which is the Holy Spirit's way of calling forth faith *and of sustaining faith*. "For the Word of the cross is folly to those who are perishing, but to us who ARE BEING SAVED it is the power of God." [1 Cor. 1:18] And this mixing invariably means speaking "Gospel" (or what we think of as Gospel) in such a way that it is not experienced by the hearers as promise. The dread of uniting law and Gospel together for Lutherans is that if there is a pathway from the one to the other, then the Gospel has ceased to be pure promise and has become instead something due, owed to our fulfilment of some condition. It is immediately rendered doubtful and uncertain.
People ask: Well, isn't faith a condition? No, faith is what appropriates, takes the promise, holds it, the trusting of it, and rejoicing in it - but such faith is never a condition for a Gospel promise. Rather than say: If you believe, your sins are forgiven you. Better to say: Your sins are forgiven you, blotted out by the blood of the Son of God. Believe it, for it is true.
Big diff between those two in how the hearer experiences them. In the first, the forgiveness is a condition that may obtain depending upon the response of faith. In the second, the forgiveness is a condition that already obtains, and which the hearer is urged to trust as so. Spoken the second way, the promise itself gives birth to or strengthens faith.
In today's homily, Pastor GeRue came to a section where the sermon soared. In it, he simply began to speak the promises by naming Christ. He was unpacking "You are the Christ, the Son of the living God." And so we had lines like: "You are the ram caught in the thicket by its horns, giving your life for ours. You are the suffering servant, pierced in your hands, your feet, your side." It kept on going and developing this "You are" theme. And these were all promises - promises fulfilled fully in Christ who is the "yea and amen" to all the promises of Sacred Scripture. Faith surges from such promises: "Yes! Yes! Yes!" it cries, "You are such a Lord for me."