What does Mark 6:50 mean?The disciples have seen something they did not expect: Jesus walking on the surface of the water, during a wind storm. As one might expect, their first reaction to this is fear. As such, "heart" here doesn't mean the blood-pumping organ. This comes from the Greek root word tharseo which means "courage or cheer." Jesus means for His miraculous appearance to give the Twelve courage and gladness. That will only be possible if their hearts are softened to His leading. Instead, their hearts are hardened (Mark 6:52) and they react in fear.
It is thought that Mark got much of his information from Peter. This may be why Mark doesn't cover one of the more remarkable events in this story. After Jesus tells the Twelve to "take heart," Peter boldly tells Jesus, "Lord, if it is you, command me to come to you on the water" (Matthew 14:28). Peter is able to take a few steps toward Jesus, but when he takes his eyes off Jesus and onto the waves, his faith falters, and he starts to sink. Jesus rescues him, and they climb back into the boat (Matthew 14:29–32). Ancient writers, particularly those of the New Testament, seem to shy away from retelling stories that might be misinterpreted as bragging.
Humans are ironic creatures. When we are threatened or confused, we tend to shut our hearts down and try to protect ourselves until we understand the situation better. The trouble comes when we regard God as part of that threat. He may speak against our lifestyle or ask us to do something we're not sure about. When we look at the circumstances, it's easy to get afraid and shut down.
We miss a lot when our hearts are hardened. When we are afraid, we miss seeing God as He is. When He has to constantly remind us to not be afraid of our surroundings or of His power and glory, it sets back our relationship and keeps us from fully joining in His work. God calls us to look at Him, instead. When we keep our eyes on Him and remember everything He has done for us, the fear goes away, and we can obey Him in confidence.