What does Mark 8:20 mean?

Jesus is teaching the disciples how to look past their stubbornly-held, preconceived ideas and the events and miracles that surround them and find the deeper personal and spiritual truths. Very shortly before, they had watched Jesus take seven loaves and a few small fish and feed four thousand Jewish and Gentile men plus women and children (Mark 8:1–10). After, the disciples had gathered seven large hampers of left-overs. Before this, they had seen a similar miracle with five thousand-plus people (Mark 6:34–44). And yet they are still anxious when they realize they have one overlooked loaf of bread to split among thirteen men.

Their response is to discuss the situation amongst themselves. It was Jesus who took the bread, thanked God for it, broke the loaves into pieces, and fed the people. It was Jesus who blessed the few small fish and had the disciples pass them out. It was Jesus who taught them that God sees all their needs (Matthew 6:32), that God will provide what they need as they seek His kingdom (Matthew 6:33), and they have only to ask and God will provide like a loving father (Matthew 7:7–11).

It's easy to scoff at the disciples for being unable or unwilling to see, hear, understand, and remember what Jesus has done for them (Mark 8:17–18), but we do the same. We forget the ways in which God has provided for us and choose to worry or feel guilty for not taking care of things ourselves. If we really believe Jesus is our Savior, we will trust Him to give us what we need. If we believe God is loving, we will trust Him to take care of us. If we believe that our commission as believers is to spread the gospel and seek God's kingdom, however, we will understand that our "needs" are a lot simpler than we thought, and may very well be covered by a single loaf of bread.