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 general command to preach only the basic doctrines, in the historical context of Alma’s community these verses have a deeper color.
Alma had been a priest of Noah and had previously taught a religion that denied the Atoning Messiah. He now teaches repentance and faith on the “Lord”—the Atoning Messiah. Alma is making sure that his people are hearing the correct message of Yahweh—as the prophets received it, as Abinadi declared it, and as Alma came to know it. When Alma commands that they preach “nothing save it were repentance and faith on the Lord,” he is telling them the most important thing to preach. It is a positive commandment, not a list of things that should not be taught. These people were acquainted with the law of Moses but had previously believed it in a version that denied the Messiah. Alma wants to make sure that his congregation understands the critical mission of this coming Messiah. Therefore, his people are to preach nothing else.