Hidden sin is sin we are unaware of.
Secret sin is sin that we know about and have covered.
Psalm 139:23-24 “Search me, O God, and know my heart; test me and know my anxious thoughts. See if there is any offensive way in me, and lead me in the way everlasting.”
The Psalmist acknowledges that even we don’t know our own hearts and we need God to search us. When we search, our perception of sin is obscured by the fact that our consciences and minds are still not what they should be.
A look at the life of David shows us that by trying to cover secret sin we fall into deeper sin.
What you cover, God will uncover. What you uncover, God will cover.
Luke 12:1-3 Meanwhile, when a crowd of many thousands had gathered, so that they were trampling on one another, Jesus began to speak first to his disciples, saying: “Be on your guard against the yeast of the Pharisees, which is hypocrisy. There is nothing concealed that will not be disclosed, or hidden that will not be made known. What you have said in the dark will be heard in the daylight, and what you have whispered in the ear in the inner rooms will be proclaimed from the roofs.”