In Alma 31: 8-9, Mormon wrote that the Zoramites had previously been taught the Nephite religion, but that they had “fallen into great errors.” That condition meant that Alma could not be sure what these poor of the land of Antionum might have been taught. After Alma’s discourse on “growing faith as a seed grows,” they were willing to believe. They were taking that first step and planting the seed. However, Alma needed to know how much to teach them.
They desired to know “whether they should believe in one God.” That is probably not a question of whether they should be polytheists or monotheists, but rather whether they should believe that Jehovah is God. That very question suggests that they had not been taught many of the fundamentals of the Nephite religion.
One thing that they had been taught is that they could not worship God unless they did so in a synagogue. That is the first thing that Alma must contradict, because he wants these people to be able to worship, but he knows that they will not be permitted to enter the Zoramite synagogues. That is the reason that he brings up the subject. Alma rests his ultimate case on the scriptures, noting that, if the Zoramite preachers have used the scriptures to teach that they could only worship in synagogues, those teachers have misunderstood the scriptures.