If Jesus' opponents were active in their unbelief (vv. 1-4), his disciples were passive in their unbelief. Unlike the Pharisees, Jesus' disciples had stayed with him and witnessed the miracles of the loaves; nevertheless, they still fail to understand his power.
Jesus warns against testing God as his opponents had just done. When Jesus warns against Pharisaic leaven (vv. 5-6), hence Pharisaic teaching (vv. 11-12), he is not implying that he disagrees with all Pharisaic teaching (23:2); the context specifies which teaching he means. The Pharisees and Sadducees have posed challenges intended to discredit Jesus (16:1-4); Jesus' words against the yeast of the Pharisees and Sadducees in this context must constitute a warning against such cynicism, which rapidly poisons the attitudes of others. Yeast was an appropriate metaphor for something that spreads; today we might employ the negative image of cancer. The disciples' passive unbelief (v. 8) suggests that the threat of Pharisaic leaven is closer to them that they would have guessed.
The disciples misunderstand Jesus' point because they are "of little faith." Perhaps they were headed for "the sparsely populated east side" of the lake, where bread would be in short supply (Hoehner 1972:204). At any rate, the disciples had inadvertently neglected their responsibility to bring bread (v. 5; see comment on 14:15), and they were so concerned about what their teacher would think about their lapse that they assumed he was addressing their own failure (16:7). Yet given what had just transpired (vv. 1-4), how could they assume that the yeast of the Pharisees and Sadducees referred to forgetting to take bread? Did they think Jesus was instructing them to bake bread from scratch once they had crossed the lake, but to make sure not to borrow yeast from the spiritually unclean religious elite? The disciples here appear inordinately dense. Thus they misunderstand because they are self-absorbed (v. 7).
But Jesus is crystal clear why they cannot understand him. Spiritual understanding cannot come apart from faith (v. 8). Had they simply forgotten to take bread-a technical rather than a moral failure-Jesus could have provided bread (vv. 9-10). That Jesus could miraculously supply bread had already eluded them twice (14:15-17; 15:33; compare 6:11, 25-34); by this point his disciples should have more faith, so he corrects them. Their real problem is that they are learning faith so slowly (compare 15:10, 16). He has serious reason for concern: these are his disciples, by definition apprentices expected to take over his earthly ministry after his departure! Yet other instances of his "little-faith" rebuke demonstrate that it represents a reproof like that of a concerned parent, not that of a harsh drill sergeant (compare 6:30; 8:26; 14:31; 17:20).