When Alma creates a church, he ordains priests. Alma’s priestly authority may have come through the king, but his authority to then create priests did not come through the king, but directly from God. Alma is creating a separate religious body with its own teachers, further defining “church” as a separate entity within a community. At this point, it is probable that many are still returning to their homes at night. Indeed, following those people will later be the means whereby Alma’s people are discovered by Noah’s spies.
When Alma has the people preach, he has them “teach nothing save it were the things which he had taught, and which had been spoken by the mouth of the holy prophets.” This sentence should be read that he told them to preach the words of the scripture as he taught them to understand it, that is, in the same way Abinadi had explained them. The priests of Noah claimed to teach the scriptures, but they denied the coming Messiah. Alma has them preach under the understanding that the scriptures looked forward to that Messiah.
When he says that they should “preach nothing save it were repentance and faith on the Lord,” he is not saying that this in the only topic, but rather that repentance and faith in the Lord were the supreme topics. The mission of the Messiah was to provide the ability to repent, thus, teaching repentance was part and parcel of teaching about the coming Messiah. Teaching of that which was to come was the whole point of the establishment of this new religious covenant.