And he charged them, saying, Take heed, beware of the leaven of the Pharisees, and of the leaven of Herod.
Jesus gives a two-fold warning, to beware of two types of teaching. While many have speculated as to what these are, Jesus says plainly what the leaven, or yeast, of the Pharisees is.
Beware ye of the leaven of the Pharisees, which is hypocrisy.
Luke 12:1 (portion)
This warning is like a warning to watch out for the “ditch on either side of the road”. We have clearly identified the one error (which is an error in teaching or doctrine), which is hypocrisy. This religious error stems from pride. The Pharisees claimed to keep the law, to be righteous legalists, but as the encounter with the woman caught in adultery proved (John 8:1-11), none of them truly were. For all have sinned and fallen short of the glory of God. Romans 3:23. In claiming to be righteous, even though each one of them knew themselves not to be, they were “actors” (the real meaning of hypocrite). While pretending to be righteous, they were each one of them filled with “dead men’s bones” (Matthew 23:27), just as we all are without Christ.