BIBLE READING: Mark 6
In chapter 6 when Mark says that the disciples’ hearts were hardened, this means the disciples did not realize Jesus’s true power. They had not taken in the true power Jesus had to feed the thousands. Jesus’s true identity was hidden to them because they did not truly grasp his.
If they had realized it, then Jesus walking on water would not have surprised them. For them, Jesus’s previous miracle of multiplying bread was not enough for them to realize just how much power and authority He had. It's like Pharaoh. When Moses’s staff became a snake, Pharaoh saw it as a simple magic trick that his magicians could easily reproduce. Therefore, his heart remained hardened to the realization of whom he was dealing with, and so he wouldn't let the Israelites go. His heart wouldn't allow the truth to come in. A hardened heart is one that is hard to be penetrated with truth or belief.