Theophylact of Ohrid
Catena Aurea · Lukasevangelium 12
Lk 12, Verses 1–3English original
In the mean time, when there were gathered together an innumerable multitude of people, insomuch that they trode one upon another, he began to say unto his disciples first of all, Beware ye of the leaven of the Pharisees, which is hypocrisy. For there is nothing covered, that shall not be revealed; neither hid, that shall not be known. Therefore whatsoever ye have spoken in darkness shall be heard in the light; and that which ye have spoken in the ear in closets shall be proclaimed upon the housetops.
He calls their hypocrisy leaven, as perverting and corrupting the intentions of the men in whom it has sprung up. For nothing so changes the characters of men as hypocrisy.