Hashem revealed himself to Abraham but did not reveal the whole of the law until he gave the Torah to Israel at Sinai.
And over time the Sages have learned more and more about the Torah, both Oral and Written, as they have studied and shared what they have learned with the rest of Israel.
Likewise Hashem has acted progressively - choosing a transient tabernacle as a dwelling and then declaring the Temple Mount as his habitation among His people.
(2) Too much focus has been put on terms of debate defined by Protestants. The faith of which the Christian Scriptures speak, pistis, does not simply mean the mental act of assenting to propositions. This leads to sterile debates about "works" or "works of the law" vs. "faith" - ZC points out the obvious: an act of mental assent to the proposition that Jesus is Lord is a "work" every bit as much as baptism is a "work."
Faith is the grace (gift) from God of being placed in a relationship with Him by Him. Faith is a relationship. When you say to your wife "I have faith in you" you are not simply saying that you assent to the assertion that she exists and that she bears a relationship toward you that requires certain mutual obligations. It is much more than that.
Since Protestant "belief" (not to mention reading the "sinner's prayer" in a tract and signing it) and mailing it in are "works," not only Luther's argument, but Paul's also, simply collapses. Since G-d insists on "works" (according to everyone but Calvinists and universalists), there is simply no safer, surer, and more objective "system" other than the Torah/Noachide laws and any further "development" is unnecessary.
(As an aside, ww, I must point out that Protestant faith is traditionally "fiduciary" and not at all "an assent to propositions." I was terribly confused shortly after my own conversion when reading an article in a rightwing Catholic publication that pointed this out, seeing as I had never been taught this before. "Intellectual faith" is still a square circle to me.)
As to "progressive revelation" in the Torah, both chr*stianity and Judaism--indeed, all religions--agree that the original religion given to Adam developed into "full bloom" at a later date. The point at issue is whether it was Sinai or Calvary that was "the fulcrum of history," verifying everything that came before and that comes after (as RaMBa"M states in his Thirteen Principals of Faith). I insist that it is Sinai that serves this purpose. For one thing, there is no remnant of a pre-Sinai religion to even dispute Sinai's claim--it is the one certain, sure, and universally acknowledged revelation. For another, the Torah has from the very beginning--from the time it was dictated to Moses, and before that, before it was written in Heaven by G-d Himself in letters of black fire on a scroll of white fire--contained the story of the Garden of Eden and Adam's sin. The Torah is the source of our knowledge of this incident. The Torah has never existed apart from the first three chapters of Genesis and the first three chapters of Genesis have never existed apart from the rest of the Torah. They are an organic whole. The "new testament," for all the symmetry of its interpretation of the incident, is an alien document that appropriated the story to create a new religion. And finally, G-d Himself in the Torah actually invoked Sinai--before it happened--as a guarantee. When Moses asked "How will I prove to the Israelites that You spoke to me when they question me about it?" G-d told him "tell them that they will serve Me on this mountain." So G-d Himself regards Sinai as the fulcrum of history.
One may speculate endlessly, but the fact of the matter is that Sinai and the Mosaic/Noachide laws emanating from it are the most certain, most objective, and safest position possible in the world and everything else (and there is no end to the claimants) is mere wind and cobwebs.