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?
Since we are now approacing Trinity Sunday, I'm adding a paragraph from Fred Craddock's contribution to Preaching Through the Christian Year B, who explains that Jesus Christ reveals the truth about God and that the Holy Spirit is the active presence of God. Then, he stresses:
But the overall affirmation of the text is that God is a life-giving God. This is no new word, as though God had ceased to be a wrathful judge and had now mellowed into forgiving love. The Hebrew Scriptures had declared God's grace in the story of the brazen serpent in Numbers 21:4-9 (vv.14-15). Our text proclaims, then, what has always been true of God, and what is comforting to hear again: God loves the world; God desires that none perish; God gives the Son that all may live; God has acted in Christ not to condemn but to save. To trust in this is to have life anew, life eternal.