Alma emphasizes the simplicity of the healing process in this incident. The people were bitten by poisonous serpents, and all they had to do was look, but many were disbelievers that something so simple and symbolic could heal anything as real and deadly as the bite of a poisonous snake. Their disbelief prevented them from looking, and prevented them from being healed.
Alma emphasizes this point because he has just told these farmers that they have also refused to “look.” Alma has pointed out that the belief in the Atoning Messiah, the “one God” is in the brass plate scriptures, but that they have failed to “see” that evidence. This people is a direct corollary to those of ancient Israel who had the source of healing in their midst, but did not “see” it. These farmers also have had the source in their midst (the teachings of Zenos and Zenock) but they have not “seen” or understood those verses.