Luke 12: 1.2.3.

Be as wise as the serpent but as gentle as the Holy Spirit. Speak the truth of God in love and do not be intimidated by any man. 

Nothing that he does in secret will not be revealed and no cover up will stay hidden.

Jesus issues a series of warnings for His disciples. Following His condemnation of the Pharisees in Luke 11, Jesus now warns His disciples not to be led astray by their hypocrisy. He then tells them not to fear those who threaten violence, but rather to fear God, who will decide their eternal destiny.

Jesus seems to be referring to the Pharisees’ hidden sins, which will be exposed at the final judgment. Faithlife Bible.

Leaven here represents the presence of corruption. NKJ Bible.

Luke 12:1–3 (NKJV)

12 In the meantime, when an innumerable multitude of people had gathered together, so that they trampled one another, He began to say to His disciples first of all, “Beware of the leaven of the Pharisees, which is hypocrisy. ² For there is nothing covered that will not be revealed, nor hidden that will not be known. ³ Therefore whatever you have spoken in the dark will be heard in the light, and what you have spoken in the ear in inner rooms will be proclaimed on the housetops.

Matthew 16:12 (NKJV)

¹² Then they understood that He did not tell them to beware of the leaven of bread, but of the doctrine of the Pharisees and Sadducees.

Matthew 23:28 (NKJV)

²⁸ Even so you also outwardly appear righteous to men, but inside you are full of hypocrisy and lawlessness.