Mark 8:16

And they reasoned among themselves, saying, It is because we have no bread.

Mark 8:16 records the disciples misunderstanding Jesus' warning because they were focused on having no bread. Their thoughts fastened on the lack of provision in the boat rather than on the spiritual danger He was pointing out. That failure to trust His power to provide, despite having seen Him feed a multitude, was unbelief.

The verse also reflects their reasoning together in sadness, doubt, and lack of comprehension after He foretold poverty, humiliation, suffering, rejection, and death at Jerusalem. Their eyes were fixed on the nearer earthly view while they clung to glowing expectations of the Messiah's kingdom and His exaltation to the throne of David. They questioned why He would voluntarily go to Jerusalem to meet cruel humiliation and death and resign Himself to such a fate. It seemed incomprehensible that the Son of God whom they loved should suffer so, and they wondered how they could face greater darkness after He had revealed Himself to them.