Matthew 16:7

It is because we have taken no bread "Neither had they more than one loaf" (Mark). It is possible that Jesus may have employed figurative language even more than was usual with Eastern teachers; certainly this special metaphorical use of leaven was new. See Lightfoot ad loc. Again, the Pharisees had rules of their own as to what kind of leaven it was lawful to use, and what kind it was right to avoid. Hence it was not strange that the disciples should imagine that their Master was laying down similar rules for their guidance.

The error of the disciples was twofold; (1) they took "leaven" in a literal sense, (2) they thought Jesus intended a rebuke to their forgetfulness. The first (1) implied a want of spiritual insight; the second (2) a want of trust.