John Lightfoot on Matthew 16:6

[Beware of the leaven of the Pharisees, &c.] There were two things, especially, which seem to have driven the disciples into a mistaken interpretation of these words, so that they understood them of leaven properly so called.

I. That they had more seldom heard leaven used for doctrine. The metaphorical use of it, indeed, was frequent among them in an ill sense, namely, for evil affections, and the naughtiness of the heart; but the use of it was more rare, if any at all, for evil doctrine.

Thus one prays: "Lord of ages, it is revealed and known before thy face that we would do thy will; but do thou subdue that which hinders: namely, the leaven which is in the lump, and the tyranny of [heathen] kingdoms." Where the Gloss is thus; "The 'leaven which is in the lump,' are evil affections, which leavens us in our hearts."

Cyrus was leavened, that is, grew worse. Sometimes it is used in a better sense; "The Rabbins say, Blessed is that judge who leaveneth his judgment." But this is not to be understood concerning doctrine, but concerning deliberation in judgment.

II. Because very exact care was taken by the Pharisaical canons, what leaven was to be used and what not; disputations occur here and there, whether heathen leaven is to be used, and whether Cuthite leaven, &c. With which caution the disciples thought that Christ armed them, when he spake concerning the leaven of the Pharisees: but withal they suspected some silent reproof for not bringing bread along with them.