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 D&C 88:6-13. It is probable that Alma’s imagery here is slightly more mundane. Keeping with the agricultural theme, “light” is visible, and real. What Alma is saying is that the swelling of the seed of faith in the portion of the word also becomes real. They will notice the evidence just as they notice light – they may not be able to feel it, nor define it, but they know that it is there, and that it is real, and valuable. This is the reason that he adds because it is discernible. It is the ability to be discerned that is the point in this verse, not the esoteric understandings of light from D&C 88.
Once they have discerned this growth, once they know that the seed is good because they can tell that it has grown, then they know that it is good, but they are still short of perfect knowledge. The are still in the process, and in the earlier stages.