Look to God and Live
Just as the children of Israel had to literally look to the brass serpent to live, Jesus Christ literally had to atone for the sins of all people for the atonement to be effective. God's justice and mercy requires it.
"Serpent Symbols and Salvation in the Ancient Near East and the Book of Mormon," Andrew C. Skinner, Journal of Book of Mormon Studies, Volume 10, no. 2
The agent of both harm and healing, death and life, is, in this instance, the serpent. The people sin and fiery serpents bite them. Moses constructs a brass image of the harmful creatures and the people are spared. But it is really Jehovah who is the cause working behind the image, the actual instigator of both death and life.
"The Whole Meaning of the Law: Christ's Vicarious Sacrifice," Jennifer C. Lane, The Gospel of Jesus Christ in the Old Testament
The truths about the Atonement affirmed in the Restoration correspond to those taught in the Old Testament, particularly those found in the law of Moses in Exodus and Leviticus and also in Isaiah’s teachings about the suffering Messiah in Isaiah 53. The substitutionary sacrifice that we see under the law of Moses is explained by Isaiah as pointing to the vicarious sacrifice of the Messiah. Together these practices and prophetic teachings can strengthen our faith in the Atonement of Christ.
"The Provocation in the Wilderness and the Rejection of Grace," M. Catherine Thomas, Sperry Symposium Classics: The Old Testament
We really begin to appreciate the Old Testament when we realize that Israel’s experiences in the wilderness are both literal and allegorical of our own experiences. Moses, speaking of manna as a symbolic teaching device, said, "[God] humbled thee, and suffered thee to hunger, and fed thee with manna . . . that he might make thee know that man doth not live by bread only, but by every word that proceedeth out of the mouth of the Lord" (Deut. 8:3).