What does John 14:27 mean?

"Not knowing" brings its own kind of terror. Experience makes people less nervous to drive cars, fly in airplanes, endure thunderstorms, or receive medical injections. The experiences, themselves, don't change. What changes is the person's expectations—they know what is happening, and what will happen next. That brings confidence and greatly reduces fear. It brings peace.

Several times in this discourse, Jesus will point out that He is preparing the disciples to "hold fast" in a difficult time (John 13:7; 14:29; 16:4, 33). This not only applies to His impending arrest and execution (Mark 8:31; John 12:34; 16:32), but to the persecution Christians will face because of their faith (John 15:18–20; 16:2–3).

The peace that Christ offers is not like that of the world. The best we can expect from the natural world is unfairness and death (Romans 8:20; James 4:14; Psalm 73:3). Even attempts to be moral, without God, lead only to frustration (2 Corinthians 7:10). Christ's "peace" here refers to a hope and reassurance that goes beyond what a fallen world can offer (Philippians 4:7). It is permanent, guaranteed, and eternal (Hebrews 6:18–19).

Here, again, Christ encourages His followers to keep their "hearts" from fear and trouble. This repeats the statement Jesus used to start this message (John 14:1), immediately after predicting Peter's cowardice (John 13:38). Here, again, scholars suggest that Jesus is distinguishing between being troubled in one's "spirit," meaning pain and unhappiness, as opposed to being troubled in one's "heart," meaning fear and despair. Jesus was said to have a troubled spirit, at times (John 11:33; 13:21). What He calls for here is not for Christians to be stone-faced and inhuman. Rather, it's to acknowledge the reality of suffering while at the same time trusting in God to make good on His promises.