What does John 16:9 mean?

Jesus has repeatedly predicted the coming of the Holy Spirit (John 14:16–17, 26; 15:26–27; 16:7). In the prior verse, He alluded to three aspects in which the Holy Spirit would "convict" the world (John 16:8). While Christians often speak of spiritual conviction (1 Corinthians 2:14–16), the literal Greek term elenxei also refers to accusation or rebuke. In that sense, the Holy Spirit will bring a form of "conviction" to the entire world. In this verse, Christ details the meaning of the Holy Spirit bringing the world conviction concerning sin.

Rejection of God, and of His Son, could be construed as the ultimate sin. The life, death, and resurrection of Jesus make God's plan clear (John 3:18, 36; 15:22, 24). The Holy Spirit will influence the lives of believers (Matthew 5:11–16) to spread that message. Followers of Christ, empowered by that Spirit, will carry Jesus' teaching around the world (Matthew 28:19). Ultimately, there is ample evidence in nature itself (Romans 1:18–20; Psalm 19:1) that people ought to seek God. Seeing and hearing the message of Christ-followers "convicts" those who ignore it (John 9:41).

This is the aspect of the Holy Spirit's "conviction" that most offends non-believers. It's one thing to suggest that Christ offers an example of good deeds (John 16:10), or that evil is in some sense going to be punished (John 16:11). What truly enrages many people is the claim that they are guilty of moral wrongdoing and are therefore subject to God's judgment. So far as believers live out the influence of the Holy Spirit, that conviction will inspire hatred and anger from the unbelieving world (1 Peter 4:3–4; John 16:1–4).