According to Brant Gardner, when Alma commands that they preach "nothing save it were repentance and faith on the Lord" (Mosiah 18:20) we must see that as a positive command, not a negative one. Alma is not telling them to stop teaching other things, but to emphasize the mission of the atoning Messiah, of the Lord that Abinadi has 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. [Brant Gardner, "Book of Mormon Commentary," [http://www.highfiber.com/~nahualli/] LDStopics/ Mosiah/Mosiah18.htm, p. 23]