Nephi gives us two important pieces of information. The first is that the Spirit has stopped him from telling them more (reinforcing again the oral nature of the discourse) and then he explains why the Spirit has so constrained him.
From what was Nephi stopped in his utterance? Nephi’s discourse was on the aspects of the doctrine of Christ that might lead to changes in performances. The clear probability here is that Nephi understands the coming fulfillment of the Law of Moses, and is unable to disclose that information to a people who will still need to be governed by the Law of Moses. In the context of this particular discourse and the direction of the logic, this appears to be the place where the discourse could have gone - and does not.
Nephi declares that the reason he cannot disclose more is the “unbelief, and the wickedness, and the ignorance, and the stiffneckedness of men.” This is a fascinating disclosure in a public discourse, because it appears to be directed to Nephi’s audience. This is the very same audience that is being told to look to the Spirit, and now are being told that “men” “will not search knowledge, nor understand great knowledge, when it is given unto them in plainness.” This is quite an indictment of his audience, his own people. Following so closely upon the baptismal covenant and the admonition that they must find their own way, we must conclude that Nephi can see that this will not be universally done among his people. Certainly there will be those who will not only enter into the way (through baptism) but who will actually receive and use the Spirit to direct their lives. It is just as clear, however, that Nephi can see that there are some (and probably many, given the clear indictment in the speech) who will not.
What does this mean for the covenant of baptism among the Nephites? What it appears to mean is that this is a voluntary covenant above and beyond the covenants of the Law of Moses that would inform all of society. As a voluntary and “higher” ordinance, it was available to those who would qualify (through repentance) but was not attached to any specific division in society nor to any rights or privileges in the society (had there been privileges, there would have been instructions on what to do). Thus this entirely voluntary action that required a personal commitment did not have the reinforcement of an entry rite into a specialized group. With so few obvious benefits, it appears that while baptism was understood, it may not have been a significant part of Nephite religious ritual until Alma associates baptism with the creation of “churches.”
We may want to presume that Nephi’s understanding of baptism and his teaching of this ordinance would have been sufficient to make it an important part of Nephite religion from this point on. Nephi’s own words suggest that this was not to be the case. Nephi’s constraint by the Spirit and his characterization of his audience as unbelievers and wicked suggests that Nephi understood that the ordinance was being given for those few who could benefit from it, but that it was destined to be a lesser part of religious practice for those “unbelievers” in his audience. Again, the very fact that Nephi is both constrained and makes this point suggests that there are significant numbers in the audience that fall into this category.
Indeed, while Nephi mentions baptism, the topic is not explicitly mentioned until we get to the story of Alma the Elder in the Book of Mosiah. A full discussion of the innovative importance of Alma the Elder will be given at that point, but suffice it here to summarize that Alma the Elder is not part of the main Zarahemla group, but is part of the group that returned to the Land of Nephi. When Alma begins baptizing, he also forms “churches.” Those innovations are brought to Zarahemla and implemented as Alma is raised to significant importance by King Mosiah. After Alma we here more of baptizing, but the concept takes a hiatus between Nephi and Alma the Elder, at least insofar as we have data in our current text of the Book of Mormon.