Sherem levels an accusation at Jacob that Jacob "keep[s] not the law of Moses which is the right way; and convert[s] the law of Moses into the worship of [Christ]." Why does Sherem make this accusation? The interpretation of the law of Moses has been behind several of the religious divisions in the history of Israel. Lawrence Schiffman describes the conditions in Israel after the Maccabean revolt (166-165 BC):
"Competing sects each sought adherents among the people. Although all were Jewish and regarded the Torah as the ultimate source of Jewish law and considered other groups' approaches illegitimate." (Schiffman, Lawrence H. Reclaiming the Dead Sea Scrolls. Doubleday, New York, 1995, p. 72).
The sects discussed are the Sadducees, Pharisees, Essenes, and the people of the Dead Sea Scrolls. While all of this division interpretation of the law occurs long after Jacob's time, it is still instructive that the law was of such central importance that its interpretation would be the basis of significant discussion and debate. The concept of the sanctity of the law and its interpretation is not hard to understand at the time of Jacob. Sherem fits into what would later be major contention among Israel, the proper reading of the law.
Why does Sherem select this point on which to debate Jacob? First because it is a significant issue. If the law is primary for the Jews (and Nephi and Jacob have clearly taught their people that they are Jews), then a significant deviance away from that law would be a major issue (and a way Sherem could claim Jacob as a false leader, which he does).
How can Sherem make such an accusation? Because, to a certain extent, it is true. Nephi before him had begun to teach the people the concept of baptism as a sign of the atonement of Christ. This is not a Mosaic practice, but rather an additional rite that Nephi added on to the list of religious observances.
Even though Nephi enjoined the people to keep the law of Moses, it was a law modified with a belief in Christ: "2 Nephi 25:24 And, notwithstanding we believe in Christ, we keep the law of Moses, and look forward with steadfastness unto Christ, until the law shall be fulfilled. 25 For, for this end was the law given; wherefore the law hath become dead unto us, and we are made alive in Christ because of our faith; yet we keep the law because of the commandments."
Jacob would have continued with Nephi's understanding and practice. Therefore, Sherem's accusation have a basis in fact that the people can see. There is the law contained in the brass plates, and very clearly additional information that has been added to it, including the additional ritual practice of baptism. These additional teachings which Nephi called making them "alive in Christ because of our faith," were precisely those Sherem could attack as additions to the written text. Sherem is making the same mistake as many modern religionists who will hold tightly to the written text, but deny the living process of revelation to God's servants.
Rhetorical: Sherem next builds another common base, the Law of Moses. Sherem accepts it, and indeed confirms it. He then places himself as a defender of the Law against Jacob as a detractor of the Law. He accuses Jacob of blasphemy, and indeed the rejection of the Law of Moses might be considered blasphemy. The evidence for this blasphemy is the teaching of a being in the far future.
While Jacob certainly gives us the "Reader's Digest" version of Sherem's argument, it is sufficient that we can reconstruct the nature of the argument. First, it depends upon acceptance of the Law of Moses, and of a general acceptance of that Law by the people of Nephi. The Law of Moses was the foundation of the people of Nephi, with their earliest religious belief and ritual governed by the Law.
After years of living the Law, Nephi and then later Jacob begin to add information to their people. As prophets who have experienced the Messiah, they were anxious to introduce an understanding of the Messiah into the worship of their people. One of the first introductions was the rite of baptism, introduced by Nephi near the end of his life. In our examination of that sermon, it is apparent that this was not only a new commandment for the Nephites, but one which had a dubious fit with the Law of Moses, for they ask Nephi what they should do (2 Nephi 32:1). How do they fit this new belief into the Law of Moses?
It may be that neither Nephi nor Jacob saw the conflict that the people saw (or that Sherem encouraged them to see). Nephi teaches that they will continue to obey the Law of Moses, but he attempts to add onto that ritual Law the observance of the gospel of Christ. During Christ's earthly mission, he resolved this problem differently, by declaring the Law of Moses fulfilled. Nephi has no such solution, only the conflict. Sherem plays directly upon that conflict, and accuses Jacob of blasphemy in attempting to diminish the Law. From a logical perspective, Sherem has placed himself in the perfect position. He has now become the defender of the true faith, and has accused Jacob of heresy for the introduction of teachings that are newer than the Law, and hard to fully incorporate in a single religious devotion.
As an argument, it is brilliantly conceived. It should have worked, and perhaps would have, except that Jacob had personal experience with the Lord on his side.