Rhetorical: In the context of Alma’s discourse to this point, he notes that the seeking of the sign is yet another hindrance to knowledge precisely because it won’t work. In this case, however, he is dealing not with the individual case, but the general case of what religion is supposed to be. Alma has a congregation that is willing to hear, and he needs to begin to teach them. At this point he begins his transition from introduction to the heart of his lesson. That less will be on the nature of faith, and it is a lesson he needs to teach this people precisely because of their circumstances on the outside of ceremonial religion. It if faith that will allow them worship outside of places and times.
Theological: Alma creates an important dichotomy. Faith and knowledge are mutually exclusive. Alma specifically notes that when “a man knoweth a thing he hath no cause to believe.” This is a critical piece of information about faith, which will be examined after verse 21 below, where the principle is repeated.