Alma suggests that the growth of the word is real “because it is light.” The imagery of light can become quite complex, as witnessed in Doctrine and Covenants 88:6-13. Because Alma is using a farming metaphor, he probably means “light” in its literal sense of being visible and real. The “swelling” of the seed of faith in the portion of the word also becomes real. Believers will notice the swelling just as they notice light; they may not be able to feel or define it, but they know that it is present, real, and valuable. This is why Alma uses “discernible.” It is faith’s ability to be discerned that is the point in this verse—not the more esoteric understandings of light from Doctrine and Covenants 88.
Once believers have discerned this growth (or know the seed has perceptibly grown), then they know that it is good but are still short of perfect knowledge.