Albert Barnes Commentary Matthew 16:12

"Then understood they that he bade them not beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees." — Matthew 16:12 (ASV)

Then understood they, etc. After this explanation, they immediately saw that he referred to their doctrines. Erroneous doctrines are like leaven in the following respects:

1. They are, at first, slight and unimportant in appearance.
2. They are insinuated into the soul unwittingly and silently, and are difficult to detect.
3. They act gradually.
4. They act most certainly.
5. They will pervade the entire soul and bring all the faculties under their control.