Speaking about the disciples confusing the leaven of the Pharisees with ordinary leaven in bread...Mark 8:21 ff
The ground of the Master's upbraiding is not that they did not understand him, but that they did not trust God; that, after all they had seen, they yet troubled themselves about bread. Because we easily imagine ourselves in want, we imagine God ready to forsake us. The miracles of Jesus were the ordinary works of his Father, wrought small and swift that we might take them in. The lesson of them was that help is always within God's reach when his children want it--their design, to show what God is--not that Jesus was God*, but that his Father was God--that is, was what he was, for no other kind of God could be, or be worth believing in, no other notion of God be worth having. The mission undertaken by the Son, was not to show himself as having all power in heaven and earth, but to reveal his Father, to show him to men such as he is, that men may know him, and knowing, trust him. It were a small boon indeed that God should forgive men, and not give himself. It would be but to give them back themselves; and less than God just as he is will not comfort men for the essential sorrow of their existence. Only God the gift can turn that sorrow into essential joy: Jesus came to give them God, who is eternal life.
....The answer then to the Lord's reproach, 'How is it that ye do not understand?' is plainly this: their minds were so full of care about the day's bread, that they could not think with simplicity about anything else; the mere mention of leaven threw them floundering afresh in the bog of their unbelief. When the Lord reminded them of what their eyes had seen, so of what he was and what God was, and of the foolishness of their care--the moment their fear was taught to look up, that moment they began to see what the former words of the Lord must have meant: their minds grew clear enough to receive and reflect in a measure their intent. The care of the disciples was care for the day, not for the morrow; the word morrow must stand for any and every point of the future. The next hour, the next moment, is as much beyond our grasp and as much in God's care, as that a hundred years away. Care for the next minute is just as foolish as care for the morrow, or for a day in the next thousand years--in neither can we do anything, in both God is doing everything. Those claims only of the morrow which have to be prepared to-day are of the duty of to-day; the moment which coincides with work to be done, is the moment to be minded; the next is nowhere till God has made it.
MacDonald, George in Unspoken Sermons Series I., II., and II.
*MacDonald's often odd sentence construction could make this seem as though he's saying Jesus wasn't God; I'm sure that's not his intention.