In the first reading God tells Moses to mount a bronze serpent so that those who looked at it were healed from deadly serpent bites.
In the Gospel reading we see Jesus in a dispute with the Jews. His listeners refused to listen to his message: they especially could not understand him when he talked about his Father in heaven. He warns them that those who persist in their sins will die and that, to be freed from sin, they must believe in him. Indeed sin and salvation are the story of our lives: we see both elements at play in our personal narratives. But it is our salvation that God desires the most: it is for our salvation that Christ came.
Jesus alludes to the bronze serpent in the desert which cured those bitten by the serpents sent by God to punish the Israelites for their lack of trust: “When you have lifted up the Son of Man, then you will know that I am He.” Just as the Israelites were saved from the serpent bites by looking at the bronze serpent, we are saved by looking at and believing in the crucified Christ.