The comment about the leaven of the Pharisees is just as much a observation of what is currently transpiring against Him as it is a doctrinal fidelity teaching. Jesus sees what is developing and the rage mounting against Him so He checks to see how far along the Disciples hearts are in the revelation process. They are currently far enough to know that He is Christ but, not so far as to know what Christ must suffer for them in the very near future. Until they grasp this further more complete revelation they will not grasp their own future pathway - the baring of their own cross for Him in His absence up until His return.
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