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.
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.
As we are now approaching Trinity Sunday, Fred Craddock (in his contribution to Preaching Through the Christian Year B) helps us as he 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 wold; 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.