Do you still not understand? Jesus must have been fuming at his disciples for their slowness at understanding issues, and by extension, their slowness to understand who their master was. Consistent with the theme running throughout his Gospel, Mark faults the disciples for remaining hard at heart even after witnessing some mighty signs performed by Jesus. He faults them for thinking/understanding/interpreting issues too literally. Jesus had just had a ‘confrontation’ with the Pharisees who had demanded a sign from him (cf. yesterday’s Gospel reading). It was a demand that visibly infuriated Jesus because, although they had just witnessed a sign from heaven, they still had the audacity to ask Jesus for a sign (as a step towards accepting him as having come from God). To Jesus, the demand for a sign was either a belittling of what he had been doing or they were testing God. Jesus termed such a demand hypocrisy since no group or persons were better placed to understand what Jesus had just done more than them. Jesus was angry at them because although they knew that what Jesus had done was a sign, they still refused to accept it as such because of their disdain for him (Jesus). And it was about this that Jesus was warning his disciples to be aware.
Like the Pharisees, the disciples were going to be better placed to identify and understand the ‘signs’ wrought by God in their midst. But unlike the Pharisees, it was going to be expected of the disciples to lead the others in identifying and appreciating the blessings of God. They were not to let petty differences or misunderstandings prevent them from giving God thanks for his goodness in the lives of God’s people. The disciples were to be on their guard lest they be tempted to attribute God’s actions to someone else, or even worse, belittle God’s works and in so doing commit a sin.