Verses 5 and 6 must be read together to make sense of Mormon's editorial addition. Mormon tells us that all Zarahemlaites were allowed to assemble together to hear the word of God. Among those in Zarahemla, the believers in the church were commanded to do so. Mormon recognizes the division in the society, and notes that the church (which was in control of the political organization) did not exclude the part of society with which they did not agree.
The contrast between the church that gathered and the law allowing all to gather tells us that the people of the church created fair laws. It may also tell us that while some chose not to assemble with the church, they might have assembled to worship their own god or gods. Mormon gives us information that further allows us to see how the contentions developed in Zarahemlaite society. Part of the speed with which they arose is that they were not directly suppressed, but allowed as part of the "liberal" laws ("…God was liberal unto all…).
Considering the social situation that will be developing, it is significant that the command to the church was to gather together in "mighty prayer in behalf of the welfare of the souls of those who knew not God." This is more than religious concern. The division between churchmen and non-churchmen would not be similar to different denominations today, but was a far more serious social problem. Alma organized an effort to convert the non-churchmen, and then an effort to pray for those who did not convert.