Mormon gives a brief description of Alma's religious teachings. While it is tempting to see the admonition to "preach nothing save it were repentance and faith on the Lord" as a command to preach only the basics, in the historical context of Alma's community, these verses have a deeper color.
Alma had been a priest of Noah, and had preached to the people a gospel that denied the atoning Messiah. Alma is making sure that what is taught among his people, in his "church" is the correct message of God, as the prophets had received it, as Abinadi had declared it, and as Alma had come to know it. When Alma commands that they preach "nothing save it were repentance and faith on the Lord" we must see that as a positive command, not a negative one. Alma is not telling to them to stop teaching other things, but to emphasize the mission of the atoning Messiah, of the Lord that Abinadi had declared so forcefully. These were people who were acquainted with the law of Moses, but who had previously believed it in a version that denied the Savior. Alma wants to make sure that his congregation understands the critical mission of this coming Savior. Therefore, his people are to preach nothing else.