That we may be healed from sin, let us now gaze on Christ crucified; for as Moses,
says He, lifted up the serpent in the wilderness, so must the Son of man be lifted up; that whosoever believes in Him may not perish, but have everlasting life.
Just as they who looked on that serpent perished not by the serpent’s bites, so they who look in faith on Christ’s death are healed from the bites of sins. But those were healed from death to temporal life; while here He says, that they may have everlasting life.
Now there is this difference between the figurative image and the real thing: the figure procured temporal life; the reality, of which that was the figure, procures eternal life.
St. Augustine of Hippo, Tractate XII.9 on the Gospel of John
Locus Communis 