Mormon tells us that although there was peace, there was not a uniformity of ideas. He gives us a single instance of one of the doctrinal differences. In this case, it is a new heresy that has been brought about by the sign of the birth of the Savior. In some way, this faction of people understood some version of the prophecy given by Abinadi:
Mosiah 13:27-35
27 And now ye have said that salvation cometh by the law of Moses. I say unto you that it is expedient that ye should keep the law of Moses as yet; but I say unto you, that the time shall come when it shall no more be expedient to keep the law of Moses.
28 And moreover, I say unto you, that salvation doth not come by the law alone; and were it not for the atonement, which God himself shall make for the sins and iniquities of his people, that they must unavoidably perish, notwithstanding the law of Moses.
29 And now I say unto you that it was expedient that there should be a law given to the children of Israel, yea, even a very strict law; for they were a stiffnecked people, quick to do iniquity, and slow to remember the Lord their God;
30 Therefore there was a law given them, yea, a law of performances and of ordinances, a law which they were to observe strictly from day to day, to keep them in remembrance of God and their duty towards him.
31 But behold, I say unto you, that all these things were types of things to come.
32 And now, did they understand the law? I say unto you, Nay, they did not all understand the law; and this because of the hardness of their hearts; for they understood not that there could not any man be saved except it were through the redemption of God.
33 For behold, did not Moses prophesy unto them concerning the coming of the Messiah, and that God should redeem his people? Yea, and even all the prophets who have prophesied ever since the world began—have they not spoken more or less concerning these things?
34 Have they not said that God himself should come down among the children of men, and take upon him the form of man, and go forth in mighty power upon the face of the earth?
35 Yea, and have they not said also that he should bring to pass the resurrection of the dead, and that he, himself, should be oppressed and afflicted?
Whether relying upon this prophecy and explanation of Abinadi, or something similar from another prophet, these people assumed that the birth of the Messiah had ushered in the time when it would no longer be expedient to live the Law of Moses. This was actually a reasonable interpretation, although Mormon indicates that it was not correct. The fulfillment of the law came through the teachings and atonement of the Messiah, not simply his birth.