We can see from the Gospel reading the fundamental mission of Jesus, which is to free humankind from their sins and to bring them to heaven. He warns all that those who persist in their sins will die and that, to be freed from their sins, they must believe in him.
Jesus alludes to the bronze fiery 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.