Charles Ellicott Commentary Matthew 23:26

"Thou blind Pharisee, cleanse first the inside of the cup and of the platter, that the outside thereof may become clean also." — Matthew 23:26 (ASV)

That the outside of them may be clean also — The implied premise is that “uncleanness” in its ethical sense was altogether distinct from the outward uncleanness with which the Pharisees identified it. If the contents of the cup were pure in their source and in their use, they made the outside “clean,” irrespective of any process of surface purification.