Those who do not see the Law as valid in the life of the Christian must also assert that this world has passed away and that the new world has already been inaugurated. This proposition must be firmly rejected because only at Christ’s coming will this world end and the new one be established.” David P. Scaer
—David P. Scaer, “Formula of Concord Article VI: The Third Use of the Law.” Concordia Theological Quarterly 42 (April 1978), 148. (HT: John Fonville)
The third use, which is the chief use.
Calvin (comments on second half of Leviticus 26):
“But if ye will not hearken unto me. Thus far a kind invitation has been set before the people in the shape of promises, in order that the observance of the Law might be rendered pleasant and agreeable; since, as we have already seen, our obedience is then only approved by God when we obey willingly. But, inasmuch as the sluggishness of our flesh has need of spurring, threatenings are also added to inspire terror, and at any rate to extort what ought to have been spontaneously performed. It may seem indeed that it may thus be inferred that threats are absurdly misplaced when applied to produce obedience to the Law, which ought to be voluntary; for he who is compelled by fear will never love God; and this is the main point in the Law. But what I have already shewn, will in some measure avail to solve this difficulty, viz., that the Law is deadly to transgressors, because it holds them tight under that condemnation from which they would wish to be released by vain presumptions; whilst threats are also useful to the children of God for a different purpose, both that they may be prepared to fear God heartily before they are regenerate, and also that, after their regeneration, their corrupt affections may be daily subdued. For although they sincerely desire to devote themselves altogether to God, still they have to contend continually with the remainders of their flesh. Thus, then, although the direct object of threats is to alarm the reprobate, still they likewise apply to believers, for the purpose of stimulating their sluggishness, inasmuch as they are not yet thoroughly regenerate, but still burdened with the remainders of sin.”
Meredith Kline–“The newness of the New Covenant does not consist in a reduction of the Covenant of Redemption to the principle of election and guaranteed blessing. Its law character is seen in this, too, that it continues to be a covenant with dual sanctions….There is no reason to regard Jeremiah’s description of the New Covenant as a comprehensive analysis, on the basis of which an exclusive judgment might then be rendered, excluding the curse sanction from a place in New Covenant administration. Even the aspect of New Covenant consummation that Jeremiah does deal with he views from the limited eschatological perspective of an Old Testament prophet….
Kline—“The theologian of today ought NOT impose on himself the visionary limitations of an Old Testament prophet. By virtue of the fuller revelation he enjoys (c/U Lk. 10:24; I Pet. 1:10-12) he is able to distinguish these two distinct stages in the history of the New Covenant and to observe plainly that the imperfection of the covenant people and program has continued on from the Old Covenant into the present phase of New Covenant history. It is in accordance with this still only semi-eschatological state of affairs that the administration of the New Covenant is presently characterized by dual sanctions, having, in particular, anathemas to pronounce and excommunications.”
http://www.meredithkline.com/klines-works/by-oath-consigned/