"And He left them, and getting into the boat again, departed to the other side. Now the disciples had forgotten to take bread, and they did not have more than one loaf with them in the boat. Then He charged them saying, 'Take heed, beware of the leaven of the Pharisees and the leaven of Herod.' And they reasoned among themselves, saying, 'It is because we have no bread.' But Jesus, being aware of it, said to them, 'Why do you reason because you have no bread? Do you not yet perceive nor understand? Is your heart still hardened? Having eyes, do you not see? And having ears, do you not hear? And do you not remember? When I broke the five loaves for the five thousand, how many baskets full of fragments did you take up?' They said to Him, 'Twelve.' 'Also, when I broke the seven for the four thousand, how many large baskets full of fragments did you take up?' And they said, 'Seven.' So He said to them, 'How is it that you do not understand?' (Mark 8:13-21).
The disciples had forgotten to take bread with them to eat. The one loaf in their possession was likely left over from the previous supply.
Jesus warned them in Mark 8:15 - "Take heed, beware of the leaven of the Pharisees and the leaven of Herod." Leaven (i.e., yeast) was used in baking bread to cause the dough to rise. It slowly worked its way throughout the dough until it permeated the lump completely. The Pharisees, Sadducees (cf. Matt. 16:6), and Herod are similar to leaven in that their influence for evil is gradual, yet powerful. Let it be understood that our Lord did not command His disciples to avoid the religious leaders or Herod (after all, they too needed to hear the truth). He merely told them to beware of their errors (cf. Matt. 23:2,3).
The disciples misunderstood the warning Jesus issued to them, thinking He was referring to the physical bread they forgot. Jesus responded to their confusion with a series of questions - "Why do you reason because you have no bread? Do you not yet perceive nor understand? Is your heart still hardened?" (Mark 8:17). According to Matthew's account, Jesus stated that their faith was little. Their hearts were still hard in that their comprehension always centered on the physical, when it should have been focused on the spiritual. Jesus can hardly believe they would think that He was rebuking them for forgetting physical bread, as if that was a problem!
Did they not remember the miraculous feedings they had witnessed? Jesus recalled the details for the feeding of the 5000 and the feeding of the 4000. It was irrelevant to Jesus that they had forgotten physical bread. He could have fed them all from a single crumb had He desired to! He was trying to teach a spiritual lesson.
"Then they understood that He did not tell them to beware of the leaven of bread, but of the doctrine of the Pharisees and Sadducees" (Matt. 16:12). Once they comprehended that He wasn't speaking about bread, then they realized that He was speaking in spiritual terms and that the reference to "leaven" was a metaphor for "doctrine." Followers of Christ today still need to beware of spiritual leaven. Sin comes in many forms, and it often affects mankind in a slow, almost unnoticeable way. However, like leaven, eventually sin permeates the whole body if we allow it to. Christians today would be wise to take heed to the things they are taught--are they really true and can they be established via the New Testament? If we do not learn from this warning, we may find ourselves ignorantly binding where God has loosed (via our traditions, like the Pharisees) or presumptuously loosing where God has bound (via our preferences, like Herod)! We must be aware of both dangers, lest we fall prey to the leaven of anti-ism or liberalism.