“Then understood they how that he bade them not beware of the leaven of bread, but of the doctrine of the Pharisees and of the Sadducees.”
Matthew 16:12 — Greek Interlinear
Church Fathers on Matthew 16:12
As the Lord had left the Pharisees on account of their unbelief, so now He teaches His disciples to be on their guard against their doctrine; whence it follows, 'And when His disciples were come to the other side, they had forgotten to take bread.'
They were bound to their Master with so great affection, that they were unwilling to part from Him for even a moment of time. And herein it should be observed how far they were from any longing for delicacies, when they took so small care for necessaries, that they had even forgotten to take bread, without which human weakness cannot support itself.
Herein the Apostles are admonished not to be partakers in the doctrine of the Jews; for the works of the Law were established to produce faith, and to prefigure the things that were to follow; and they on whose times truth itself had chanced should look for no further types of truth; lest the teaching of the Pharisees, which knew not of Christ, should stay the effect of Gospel truth.