As Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up, that whoever believes in him should not perish, but have eternal life.
God had delivered the people of Israel from bondage in Egypt in ancient times. But on the journey to the land of Canaan Israel became dissatisfied in the wilderness and rebelled against God. God then sent poisonous snakes whose bites caused them stinging pains. There was great dismay, for many died from the serpents' venom.
For those doomed to death God had a very simple life-saving message. He instructed Moses,
“Make a fiery serpent, and set it on a pole; and it shall be that everyone who is bitten, when he looks at it, shall live” (Numbers 21, 8). In other words, "Don’t look at the bite or think of the danger, look at the sign that God has given!" Salvation and help lay solely in looking at the brazen serpent simply because God guaranteed it with His word.
Maybe some were contemptuous and considered such a measure irrational. How could a glance at a piece of bronze possibly help? Perhaps they tried their own means: sucking out the poisoned blood etc.. All those precautions of their own devising proved useless, however: they died! But others obeyed God, looked at the sign set up there and remained alive.
For us it is a question of looking to the cross of Jesus. There the Son of God died as the substitute for the guilt of lost sinners. Only by looking to Him in faith can we obtain forgiveness and eternal life.