As we have been reading (slowly) through Mark's account of Jesus' life, we have seen Jesus doing many things that show He is from God. For the most part, these unbelievable things have been mostly accepted. We haven't heard many negative reactions to Jesus up to this point.
As we get into chapter 6, though, that begins to change. Jesus goes back to Nazareth, where He was raised, and the people there doubt Him. After all, they had seen Him grow up. Who does this guy think he is? Isn't he the carpenter's son? And they didn't believe Him. These people that had known Him all His life didn't see how God was at work in His Son growing up right among them.
Then Jesus sends out His apostles, giving them some of His authority over illness and the spirits of this world. With all these things happening, Jesus was getting noticed, even by the rulers. Herod thought Jesus was John the Baptist come back to life, and then we get a quick aside as to how John met his end. Herod started to see something ominous in Jesus, and it didn't exactly make him want to get closer to Jesus.
Following that, we then get one of the more popular stories of Jesus' life, the feeding of the five thousand men (likely plus women and children) with a mere five loaves of bread and two fish. However Jesus brought this miraculous feeding about, it was definitely a reminder of how God had fed the Israelites with bread and meat (manna and quail) while they were in the wilderness. Jesus provides yet another connection back to what He was doing with and through His people even back then.
Then Jesus walks on water while the disciples are battling the wind in their boat, and people come flocking to Jesus to be healed once He reached the other side of the sea. Even in this, the disciples are unsure of what they see as they see Jesus walking toward them on the sea. They are being kept from recognizing all that God is doing while wrapped up in human flesh.
In this chapter, we see some of the challenge of Jesus. Jesus is hard to believe. How can someone who was born and grew up be doing these great signs from God? Is He someone that has lived before, a powerful man of God that has been raised as a sign to God's people? How can He feed so many people with so few resources, and why didn't Mark or the other writers of Jesus' story explain how those loaves and fish kept on coming? Why did people not believe, even when they had seen Him do the same thing for others?
The challenge of Jesus is no less for us now. Sure, we might think that if Jesus were walking around us, we might believe more readily. But given the sheer number of people we encounter in His stories who struggle with the challenge of Jesus when they witnessed these things, why do we think we would be different? I would dare suggest that this is why God has to be the one who creates faith, because we humans simply have a hard time believing anything that we cannot prove. Jesus was challenging then, and is no less challenging now, and that is why we cling to God's gift of belief and faith.