I ran across an excellent statement by theologian Gerhard Ebeling about the importance of the distinction between the law and the gospel in his book, Luther: An Introduction to His thought:

 “What does ‘distinction’ really mean here? Obviously it does not mean a division, a separation. There is not merely an alternative: either the law or the gospel.  It is not as though one can be replaced by the other.  In a series of passionate conflicts, Luther attacked the antinomian misunderstanding which alleged that because Christian preaching was the gospel, it had nothing to do with the law.  Moreover, he regarded it as obviously necessary, for the sake of the purity of the gospel, to defend the law.  If the gospel were meant simply to compete with and replace the law, it would itself be no more than a form of law.  But whereas the relationship is not such that within it one simply excludes the other, like light and darkness or good an devil, neither is it accurate to consider both as complementary, as though the law were not sufficient, and the gospel had to be added to it, or as though the gospel alone were not sufficient, and the law was required as well.  The need for a proper distinction seems to face us with a task which is more difficult than that of mere separation or mere association.  This task is to maintain an opposition between the two which is of the nature of a mortal enmity—so that the law slays the gospel, and the gospel the law—but also, at one and the same time, to reduce this enmity to order, by bringing both into a proper relationship, in which each remains in its own place and within its own limits: the law does not claim to be the gospel, and the gospel does not attempt to take over the role of the law.

According to Ebeling, the law and the gospel are different and are differentiated against each other. But it’s not as if the law and the gospel are two halves, brought together to form a single whole; they are each, in themselves, wholes. Ebeling makes an important point: the law is not lacking anything and thus needing the gospel added to it, and vice versa. The law and the gospel are fully functional and competent in their own right. What the law lacks is not something that renders it ¾ or ½ of its wholeness, but rather is a product of its wholeness: it is static, in its entirety.

We don’t add the gospel to the law to make the law do what it just flat out cannot do: it can only demand and reward or condemn, but it cannot move the hearer. The gospel is not God’s way of revamping or refurbishing the law. The gospel is its own word with its own function. Rather than static, the gospel is dynamic, in its entirety. The gospel can do what the law cannot: move the hearer to faith and in to the love of God. And, thus, why the distinction between the law and the gospel is so important: the hearer must hear the first word and then the second clearly distinct from the first. Each word must be kept and held in its rightful place. And, (contra Barth), when the two words are confused or blended or mistaken for the other, then neither is rightfully heard and both become ineffectual in their task and the hearer is left with nothing, more than that they are left without salvation.

 “Christian preaching is the process in which the distinction between the law and the gospel takes place.  This must not be misunderstood.  The purpose of Christian preaching is not primarily that of instruction concerning the distinction between the law and the gospel—though it is so in a secondary sense. Rather, the concern of Christian preaching is to put into practice the distinction between the law and the gospel, that is, to carry on the progress of a battle, in which time and again the distinction between the law and the gospel is newly at issue and is made in practice.  This means that making the distinction in practice between the law and the gospel is not fortuitous and incidental to the process of preaching, but is what is really meant to take place within it.  But if the process of preaching is what it claims to be, that is, the process of salvation, then as the distinction is made between the law and the gospel, so the event of salvation takes place. And a confusion of the two is not a misfortune of little significance, a regrettable weakness, but is evil in the strict sense, the total opposite of salvation.