What does John 8:21 mean?
Jesus is debating with the Pharisees, standing in the temple during the Feast of Booths (John 8:12–20). Earlier, as Jesus taught the crowd, these same men had sent temple guards to arrest Him (John 7:32). Instead, the guards came back speaking of Jesus' profound words (John 7:45–46). Those men were, in part, stymied by Jesus' comments about leaving to a place where others could not follow Him (John 7:33–36). In this verse, Jesus makes a very similar remark by telling the Pharisees that He will be going somewhere they cannot go.As with His prior comments, this might have made the religious leaders hope that Jesus intended to leave Jerusalem. It also seems to have been interpreted as a reference to suicide (John 8:22). It was not entirely uncommon for philosophers and spiritual gurus to commit suicide, and this would have been perfectly fine with the Pharisees.
However, in this case, Jesus specifically condemns the Pharisees and their lack of faith. Sooner or later, their time to accept Christ would end, and even though they might seek Him, they'd be damned for their sins. Later, Jesus will re-emphasize this point, telling those who reject Him that they are destined to die if they remain in their spiritual stubbornness (John 8:24).