What is here spoken of false prophets we may apply to all whose dress and speech promise one thing, and their actions exhibit another. But it is specially to be understood of heretics, who by observing temperance, chastity, and fasting, surround themselves as it were with a garment of sanctity, but inasmuch as their hearts within them are poisoned, they deceive the souls of the more simple brethren.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).