I would argue that Republication is taught by Paul in Galatians 3. The Judaizers viewed the Mosaic covenant as an addition to the Abrahamic covenant showing, as claimed, that the blessing must be received by obeying the Mosaic law. But Paul corrects them by explaining that the Abrahamic covenant is fundamentally a gracious, promissory covenant in which the blessing is received by faith. Although the Mosaic covenant (“the law”) came 430 years after the promise, it did not annul the prior Abrahamic covenant of promise ratified by God’s self-maledictory oath. Why then the law? It was “added” (top layer) until the Seed should come, who would be born under it to bear its curse and fulfill its demands in our place, thereby securing the blessing.
Republication depends on the notion that the old covenant order was composed of two layers: the national-typological (upper) layer and the personal-soteriological (lower) layer. At the lower layer, the old testament saints were saved by faith in the Messiah to come as part of the one covenant of grace that begins with the first promise of the gospel after the Fall (Gen 3:15) and continues unbroken into the new covenant. The works principle in the old covenant, the probationary arrangement akin to the Adamic covenant of works, was confined to the upper, national-typological layer. National Israel, as a corporate Adam, was on probation in the land. Israel had to be righteous in order to retain the land. But where does Kline get this idea of the two layers? It comes from the concept of two-level fulfillment.
And what about the problem that Israel was far from being perfectly righteous and yet retained the land for a long time prior to the exile? This issue is explained by Vos and Kline in terms of the concept of typological legibility:
Vos: “The theocracy typified nothing short of the perfected kingdom of God, the consummate state of Heaven .... Law-observance, if not the ground for receiving, is yet made the ground for retention of the privileges inherited. Here it can not, of course, be denied that a real connection exists .... The connection ... belongs not to the legal sphere of merit, but to the symbolico-typical sphere of appropriateness of expression. As stated above, the abode of Israel in Canaan typified the heavenly, perfected state of God’s people. Under these circumstances the ideal of absolute conformity to God’s law of legal holiness had to be upheld .... The requirement could not be lowered. When apostasy on a general scale took place, they could not remain in the promised land” (Biblical Theology, 126-27).
Kline: “The typological objective in the case of the Israelite kingdom was to teach that righteousness and prosperity will be conjoined in the consummated kingdom. For the purpose of keeping that symbolic message readable, persistent wholesale apostasy could not be allowed to accompany possession of the promised inheritance. But, on the other hand, the pedagogical point of the typological arrangement could be satisfactorily made, in a positive fashion, in spite of the inevitable imperfections of the people individually and as a nation. In meting out the blessings and curses of the Mosaic Covenant, the Lord applied the standard of symbolical appropriateness or typological legibility” (Kingdom Prologue, 239-40).
Republication is a debated topic today and is often misunderstood. The idea is not that the Adamic covenant of works was republished for Israel at Sinai without change. Kline argues that national Israel was “under probation in a covenant of works” (Kingdom Prologue, 352), not “the” covenant of works. That is why I pefer to speak of “typological republication.” The key to making sense of this is to recognize that the Mosaic economy was complex. More things are going on there than simply administering the one covenant of grace. According to Kline, “The old covenant order was composed of two strata,” and “The works principle in the Mosaic order was confined to the typological sphere of the provisional earthly kingdom which was superimposed as a secondary overlay on the foundational stratum” (KP 321). The foundational layer, the one covenant of grace, continues unbroken from Genesis 3:15 on. But at the secondary layer, a works principle is reenacted with national Israel in order to set the covenantal context for the exile and the coming of Christ, who was born under the law that he might perfectly fulfill it and win the eternal inheritance for us.