What does John 11:15 mean?

Jesus sent back an encouraging response when He was given news that Lazarus was sick (John 11:4). He then waited two days before announcing His intent to return to Judea, where men were seeking to kill Him (John 11:6–8). The disciples were understandably confused. Not only did Jesus tell Lazarus' sisters not to worry about death, He also said that Lazarus was asleep (John 11:11). In the prior verse, Jesus explained to the thick-headed men that Lazarus was dead.

Jesus notes that this is a good thing, at least as it applies to the disciples. This will become another example to strengthen their faith. Despite their confusion and ignorance, these men have been loyal to Jesus even when others left (John 6:66–69). That loyalty will once again be rewarded as they will witness the most spectacular of the seven "signs" recorded in the gospel of John.

The disciples' devotion to Jesus doesn't mean they're enthusiastic about this decision. In the next verse Thomas, infamous as a "doubter" (John 20:24–29), suggests that they might as well go along so they can all die with Jesus. Sarcastic or not, the threat was real, and Jesus' followers are willing to face it.