These verses mark a turn in the teaching. It is important to understand that the very first thing that Christ does, after demonstrating who he was, was to personally authorize those who could baptize. Baptism was the way to accept Christ’s atoning mission. Therefore, as they had clearly accepted Him, they needed to demonstrate and covenant that acceptance through baptism.
After that, there are other things to be taught, and Christ uses the problem of disputations as the pivot. There have been times of peace among the Nephites, but often the ending of that peace was initiated by disputes. Christ declares that “the spirit of contention is not of me.” That doesn’t mean there should be no discussion, nor learning of new things through asking questions. The problem is “the spirit of contention,” which is a fault of the soul. It is anger in the questioning that does not lead to peace of the spirit.
Verses 30 and 31 make the pivot through a contrast: “[T]his is not my doctrine” is contrasted to “I will declare … my doctrine.”