Jesus has been talking to Nicodemus, but now is speaking to a plural you.
Jesus says to his hearers, "The Son of Man must be lifted up so that whoever believes in him may have eternal life."
We take "lifted up" to mean the crucifixion or the resurrection or the ascension, or all of these. He is in Jerusalem at the beginning of his ministry yet his words will be understandable after his death, resurrection, and ascension.
Or, will they be? Nicodemus had seen signs as had the other Pharisees but he was unwilling to come publicly to Jesus. The audience for John's Gospel had seen even more signs; were they able to believe?
Jesus said that those who believe in him may have eternal life; also see, John 3:36; 4:14; 5:24; 6:27: and 17:14 (with thanks to The New Interpreter's Study Bible).
What is being promised? Not just heaven later after we're dead. The word we translate as eternal carries the meaning of a different quality of life, a new life free of the worldly, temporal concerns of the old life.
[Jesus alludes to Moses' lifting up a serpent in the wilderness, Numbers 21:4-9, a passage that I will discuss Wednesday.]
Surely, all football fans have seen that sign in the stands saying John 3:16. Please don't stop with that verse. God's intention is that this eternal life is for us all.
Lectio Divina: John 3:17