What does John 15:21 mean?

Those who commit crimes or sins can expect the natural consequences of those actions (1 Peter 4:14–15). A person fired from a job or thrown in jail as a result of theft cannot claim they are being "persecuted," especially not for their Christian faith. People who attach themselves to the word "Christian" (John 15:2), but who disobey the teachings of Jesus, are not being hated for the sake of Christianity; they're suffering for the things they've done contrary to the commands of Christ.

Christ's point here is about someone who acts in a Christlike way and the unbelieving world attacks them because they are imitating Christ. This is the instinctive reaction of all people controlled by their sin nature (1 Peter 4:4).

In public discussions with His critics, Jesus pointed out that His detractors did not know God (John 8:55). That was the reason for their disbelief and hatred (John 8:43–44). Those who are separated from God don't want to know the truth. They become angry when confronted with it (2 Corinthians 4:3–4; Ephesians 4:17–19). Even when Christians act in love and goodness (John 14:15; Matthew 5:16), they should not be surprised to see the unbelieving world react with spite.

Jesus' motives for making these remarks is not to frighten the disciples. Rather, it's to insulate them against the struggles they are about to endure. Soon, Jesus will be arrested and crucified (John 18:1–3; 19:18). The disciples will see the brutal opposition thrown against the early church (Acts 8:1–3). Knowing, in advance, that all of this is part of Christ's knowledge and understanding (John 13:19; 14:25, 29; 16:4) gives believers confidence to "hold fast" to faith (Hebrews 4:14–16; 12:1–3).