First, let it be noted that Daniel Fuller has MANY problems. He denies plenary inerrancy of Scripture. In addition, he appears to back away from the historic Protestant stance on justification by faith alone.
Second as to his book it has been looked at and already refuted. You may begin by reading,
Covenant Theology under Attack, by Meredith G. Kline.
Third, you may enjoy reading Sam Waldron's doctoral dissertation,
Faith, Obedience, and Justification: Current Evangelical Departures from "Sola Fide, where he exposes how Daniel Fuller, Norman Shepherd, and Don Garlington have deviated from the historic Reformation doctrine of justification by faith alone.
Fourth, your claim that, “So if we apply Genesis 9:6 to Cain the murderer, then we're asking God to take vengeance on us sevenfold. This clearly doesn't make any sense. It makes sense to understand Genesis 9:6 as applicable to post-diluvian humanity, but not to antediluvian humanity,” is not accurate. You fail to correctly understand what Analogy of Faith entails. By definition it means:
Analogia Scripturae: The Analogy of Scripture. This is explained in the formula: "Scripturam ex Scriptura explicandam esse", or "Scripture is to be explained by Scripture." Related to this principle is the principle of Analogia Fide, or "Analogy of Faith." That is, Biblical doctrines are to be interpreted in relation to the basic message of the Bible, the Gospel, the content of faith, often called The Faith. Cf.1 Cor.2:13, 15:1-4.
As one may note the Analogy of Faith includes, literary structure, original meaning, and modern application, etc. in its examination of the original text. If one cannot establish how the Analogy of Faith is relevant to the original meaning then how may they know if it is being properly applied today?
Can Genesis 9:6 be analyzed in reference to this definition? Of course it can and it has relevance for us today, as well for them of yesterday (before the flood). Genesis 9:6 is actually consistent with Genesis 4:15 (and yes, there are dispensational discontinuities in every dispensation, but we must also fully regard the consistencies).
Genesis 9:6 Whoso sheddeth man's blood, by man shall his blood be shed: For in the image of God made he man.
Genesis 4:15 And Jehovah said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And Jehovah appointed a sign for Cain, lest any finding him should smite him.
Do you really think there were no commandments regulating society prior to Moses writing them down? If so, what right did God have for approaching Cain for his sin? The Scripture says the law was written upon our very hearts (Rom. 2:15). In addition, many of the Mosaic laws are only "very briefly" worded, and are almost unintelligible without certain presuppositions which were assumed to be generally held. Moreover, the lives of Jacob and Joseph's brothers reveal they "knew" they had done wrong prior to the written law of Moses. Adam himself did not see the written law of Moses, but "knew" he had sinned both by a (1) convicted heart, and (2) the VERBAL, not written, law of God (Gen. 3:1 says,
hath God said,
not hath God written).
The fact that Genesis 9:6 was already in effect is proven by the fact that God “appointed a sign for Cain, lest any finding him should smite him,” otherwise he would have been killed before accomplishing God’s will. Post-flood Genesis 9:6 screams at us to look once again at pre-flood Eden when it uses the phrase, "For in the image of God made he man" (compare, Gen. 1:26-27). Furthermore, where do you think the definition of murder came from so post-flood humanity could understand Genesis 9:6 -- pre-flood humanity. As far as the "sevenfold" cursing I could say that this was carried forth as well--read imprecatory Psalm 79 or Lev. 26:28, however, it would also be true that in Cain's case God made an exception to His normal law (God also kept Pharaoh alive for many years though he murdered Israelites-Rom 9:14-18, etc.), so His divine purpose could be fulfilled in Cain. And then we could raise up the question of what "sevenfold" vengence is. Does this mean to the seventh generation as Luther taught or does it mean complete, full and absolute justice (K&D; John Currid; Bruce Waltke, etc.)? As Willet comments, "he would be visited more severely than Cain, as being guilty not alone of homicide, but of transgressing the Divine commandment which said that Cain was to live (Willet, per Spence of
The Pulpit Commentary). Genesis 4:15 is relevant because it reveals that Cain feared man and not God (compare, Matt.10:28). I would continue but IMO Genesis 9:6 is relevant to antediluvian humanity and post-diluvian humanity!
Lastly, you may find it helpful to read several books on hermeneutics from a Reformed perspective: "
He Gave Us Stories, The Student’s Guide to Interpreting Old Testament Narratives," by Richard L. Pratt, Jr. would be an excellent beginning point.
(sorry, but I had to repost this as the first post was not being seen by everybody--anomaly?)