In today’s gospel, Jesus almost loses patience with his apostles because of their lack of faith in him. Concern about empty stomachs makes them forget who it is who has called and cared for them until now. What Jesus probably wants here is not a blind faith that will work a miracle for them, but that they at least begin to glimpse the spiritual nature of his mission, to which the miracles point.

In the language of the Pharisees, yeast (or leaven was regarded as a source of impurity, and symbolized man’s evil tendencies. So when Jesus warns his disciples against the yeast of the Pharisees, Jesus is being critical of the bad attitudes that the Pharisees had towards him in their refusal believe in him.

Jesus is in fact giving his disciples a serious warning here. They are
acting no differently from and are just as obstinate as his opponents.
In this case they are anxious about bread. What short memories they
have. They have already forgotten how he broke and shared five loaves
of bread among five thousand people and many pieces were left over.
They have not yet understood that the bread broken and shared was the
bread of life.

As we worship God today, let us pray for the grace to recognize the
greatness of the mission with which Jesus wants us to be associated.