“As they were gathering in Galilee, Jesus said to them, “The Son of Man is about to be delivered into the hands of men, and they will kill him, and he will be raised on the third day.” And they were greatly distressed.” (Matthew 17:22–23, ESV)
Hearing of Jesus' ordeal to come, the disciples were "greatly distressed." Though Jesus meant to prepare them and even build their faith by telling them ahead of time (e.g. John 14:29), they were still shaken.
The depth of the sorrow to come overwhelmed them and they missed something they should have had.
When Jesus promised the disciples that affliction was coming, he wasn't doing so passively. He wasn't a mere spectator suffering the slings and arrows of outrageous fortune. His suffering was actually just another event under his sovereign control. His promise of trials to come were, at their root, another reminder that he was in control. Only the almighty can promise and truly guarantee the result.
Many Christians are suffering through trials right now, trials that Christ promised so many years ago (e.g. John 15:20, Luke 14:27). I don't think we are supposed to celebrate these trials, but the promise of our Lord reminds us that we are still secure in his hands.
Though the world may rage, though your sorrow may be deep–Christian, you are still His.