What does John 14:28 mean?
Here, Jesus openly declares that these men do not fully understand what they've been taught. They've heard Jesus speak of leaving, of going to the Father, and so forth. He's made mention of being captured and killed (Mark 8:31; Luke 9:22; John 12:32–33). At best, the disciples miss the point entirely (Mark 9:31–32). At worst, they try to rebuke Jesus (Matthew 16:21–22).This is not a criticism from Jesus—He is not reprimanding the disciples for being sad or unsettled in this uncertain time. Scripture often notes that the disciples do not fully grasp what Jesus teaches until after they see the full picture of His ministry (John 2:18–22; 12:16; Luke 24:6–8). Jesus has noted that understanding requires the influence of the Holy Spirit (John 14:16–17; 14:26).
Instead, this is both a teaching point and a moment of encouragement. It's not entirely different from telling someone, "when this is all over, you'll see that you had nothing to worry about." With the benefit of hindsight, it's possible to see Jesus' sacrifice as part of His glorification, which also glorifies God (John 13:31–32) and brings about the salvation of believers (Hebrews 2:10).