Alma is referring to the brazen serpent which Moses made to symbolize Christ.
’And the people spake against God, and against Moses, Wherefore have ye brought us up out of Egypt to die in the wilderness? for there is no bread, neither is there any water; and our soul loatheth this light bread.
’And the LORD sent fiery (poisonous) serpents among the people, and they bit the people; and much people of Israel died.
’Therefore the people came to Moses, and said, We have sinned, for we have spoken against the LORD, and against thee; pray unto the LORD, that he take away the serpents from us. And Moses prayed for the people.
’And the LORD said unto Moses, Make thee a fiery serpent, and set it upon a pole: and it shall come to pass, that every one that is bitten, when he looketh upon it, shall live.
’And Moses made a serpent of brass, and put it upon a pole, and it came to pass, that if a serpent had bitten any man, when he beheld the serpent of brass, he lived.’ (Numbers 21:5-9)
Only from Nephi do we learn that there were many who were bitten who were too stubborn to be healed, the labor which they had to perform was to look; and because of the simpleness of the way, or the easiness of it, there were many who perished (1 Nephi 17:41). This stubbornness is what astounded Alma, O my brethren, if ye could be healed by merely casting about your eyes that ye might be healed, would ye not behold quickly, or would ye rather harden your hearts in unbelief, and be slothful, that ye would not cast about your eyes, that ye might perish? (v. 21)
Alma also underscores the fact that they did not understand the symbolism of the brazen serpent, and this because of the hardness of their hearts. John taught us what the serpent was to represent, as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up: That whosoever believeth in him should not perish, but have eternal life (Jn 3:14-15, see also Hel 8:15).
As the Israelites were suffering from physical sickness, they were given the opportunity to be healed, almost in an instant, and yet they declined. Similarly, the stubborn sinner will often wallow in spiritual sickness rather than look upon the Son of God with faith, having a contrite spirit (Hel 8:15). The spiritually ill continue to suffer (and are in danger of spiritual death) because they do not believe that [the Lord] would heal them (v. 20).
Boyd K. Packer
"’How silly,’ some must have said. ’How can such a thing cure me? I’ll not show my stupidity by paying any attention,’ and some would not look….
“And today many say, ’How silly! How could accepting Christ save me?’ They will not turn their heads to look nor incline their ears to hear. They ignore the great witness that comes from these conferences. We ought to, indeed we must, heed the counsel of these men, for the Lord said, ’What I the Lord have spoken, I have spoken, and I excuse not myself; and though the heavens and the earth pass away my word shall not pass away, but shall all be fulfilled, whether by mine own voice or by the voice of my servants, it is the same.’ (DC 1:38)” (Conference Report, Oct. 1968, p. 76)
Harold B. Lee
“Some may say all of what I have said sounds so simple. It is. It is like the rod of Moses on which the serpent-bitten Israelites had only to look to be healed. But, as the Book of Mormon reminds us, ’because of the simpleness of the way, or the easiness of it, there were many who perished’ (1 Nephi 17:41). Strange as it seems, some men are, as Jacob described them, forever ‘looking beyond the mark’ (Jacob 4:14), missing the plain and simple truths in their search for complexity!” (Teachings of Harold B. Lee, p. 334)
Neal A. Maxwell
"Many today do not believe, of their problems, that the gospel of Jesus Christ will heal them. But they are wrong. Many today are so hardened that they will not even look, but one day their knees, too, will bow in acknowledgment that Jesus is the Christ.
"Again, the issue is the importance of being able to believe in the truth in order to do the most realistic thing to spare us present pain and spiritual death.
"Symbolically and actually, the Son of God has been lifted up upon the cross, and it is to him that we look so that we might ’live, even unto that life which is eternal.’
“Each family, properly established, can help itself and others to do that simple thing.” (That My Family Should Partake, p. 97)