What does Mark 11:21 mean?
Much of what Jesus does in His ministry isn't intended primarily for that specific moment. Rather, He often acts so that the disciples will have a vivid lesson, example, or application available later. Jesus first cleanses the temple courtyards of money changers and livestock merchants early on, shortly after His first miracle. The religious leaders confront Him, asking on what authority is He acting. He tells them that if the temple was destroyed, He would built it again in three days. It isn't until after the resurrection that the disciples realize He is talking about Himself (John 2:13–22).The night of the Passover, Jesus tells Peter that before the cock crows Peter will deny Him three times. Peter doesn't remember these words while he is in courtyard of the high priest. He only remembers after he has denied Jesus (Mark 14:66–72).
After the crucifixion, when the women go to the tomb to tend Jesus' body, the angel reminds them that Jesus said He would be crucified and raised on the third day. Then the women remember His words (Luke 24:1–8).
After Jesus received His glorified body, the disciples remember the prophecy from Zechariah 9:9 that the Messiah will ride in on the colt of a donkey (Mark 11:7; John 12:16).
Sometimes, Jesus must spell out what has happened. When the disciples forget to bring bread on their travels, Jesus reminds them how He fed two crowds with very few loaves and fishes (Mark 8:14–20). After the resurrection, He meets two disciples headed to the village Emmaus. Jesus spends all evening with them, explaining how ever since Moses the Scriptures have pointed to the death of the Messiah (Luke 24:13–27).
The reason the disciples don't understand in the moment varies. Sometimes they harden their own hearts (Mark 6:52), while in other cases God hides the truth from them until it will impact them in a greater way (Luke 18:34).
Every Scripture is useful for teaching (2 Timothy 3:16–17), but Scripture is also useful for validating itself. Hundreds of prophecies have already been fulfilled. In a broader sense, the truths of Scripture reveal themselves in our own lives. We might read something in the Bible without understanding it. Later, we may find we've lived through an experience that reveals the truth of what we've previously read: evidence that the Bible is God's Word and something we can trust.
