The feeding of the five thousand begins another unit in Mark stretching from 6:30-8:21. This unit is bounded by the two feeding stories. The theme, of course, is understanding. Have they processed enough about Jesus' identity and authority to understand what he is doing and who he is? The two stories, back to back, about the feeding of the five thousand and Jesus walking on water represent a new exodus that Jesus is leading the people through. The feeding takes place in a desert (6:35) and recalls God's feeding of the people through manna. Except here, it is Jesus who feeds them (the Lord). Later, he walks on water, which recalls the parting of the waters when Israel escaped Egypt. The stories are concluded by healings, which bring the wholeness to God's people that God intended. Chapter 7, to get ahead a little bit, will detail a reckoning about the purpose of the law, a blessing to the nations through the faith of a Gentile woman, and a prophetic-style healing. All this occurs before the second feeding, which will conclude with the disciples' confusion and Jesus' question: How is it you do not understand?" (8:21).
In chapter 6, this misunderstanding comes in when the disciples see Jesus walking on the water and believe they have seen a ghost. Mark concludes that they had not understood about the loaves, because their hearts were hard. Mark is showing us that Jesus can be elusive and our discipleship is more difficult than we think; we should not take it for granted. No sooner has Jesus made clear his identity and his authority in a recreation of sorts of the exodus and his own disciples do not understand!
Do not underestimate your ability to delude yourself with a hard heart. Following Jesus is not about the intellect; it is about the transformation that occurs in the heart. If even the disciples, who were present with Jesus for all of this and yet had hard hearts that blocked spiritual perception, how much more must we guard our own hearts and practice the disciplines of word and prayer to remain open to following Jesus discipleship?