For he that takes heed of the leaven of the Pharisees and the Sadducees, does not observe the precepts of the Law and of the letter, and neglects the traditions of men that he may do the commandments of God. This is the leaven of which the Apostle speaks, “A little leaven leaveneth the whole lump.” [1 Cor 5:6]
By every means also we should avoid that leaven which Marcion, Valentinus, and all the heretics had. For the nature of leaven is such, that when mixed with flour, that which seemed a little increases to a large quantity, and brings the whole mixture to its own flavour. Thus heretical doctrine if it have cast but a small spark into your breast, in a short time a mighty flame is raised, and drives the whole temper of the man along with it.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).