Summary: One of Satan’s biggest lies is that there are some sins that God does not either care about or know about.
Purpose: To explain the folly and danger of trying to hide our sin.
Aim: I want the listener to repent of and forsake sin of every kind.
INTRODUCTION: One of Satan's biggest lies is that there are some sins that God does not either care about or know about.
"In the Scottish highlands is an old bridge that spans a wild waterfall. Its structure is so massive, and it rises so high above the gorge that it is known as "The High Bridge." But something happened that made it necessary for officials to condemn it. A tiny birch seed, caught by a gust of wind, dropped into a small crevice above the keystone. It lodged unnoticed in the lime, and before long it germinated. Soon it was a young sapling, but still nobody saw it. As it grew into a tree, its roots went deep into the mortar. Eventually it began to loosen and crack the masonry so that the arch was severely damaged. The bridge that had defied violent storms and supported the weight of marching armies finally had to be closed to traffic. It had succumbed to a small seed.
"So too in the Christian life, one little hidden sin can weaken the foundation of a person's character and be the cause of his downfall. I think David must have sensed this when he cried out, 'Cleanse Thou me from secret faults.'" 
I. In What Ways Can Sin be Secret?
A. Our sins may be hidden from us
Before Paul became a follower of Christ he said he that "even though I was formerly a blasphemer and a persecutor and a violent aggressor. Yet I was shown mercy because I acted ignorantly in unbelief" (1 Timothy 1:13, NAS).
It is very possible to sin and against God and not know that it is a sin. God has given us a written revelation that tells us what we need to know about sin. Ignorance is never an excuse to a Christian.
B. Our sins may be hidden from others
"The sons of Israel did things secretly which were not right against the LORD their God. Moreover, they built for themselves high places in all their towns, from watchtower to fortified city. " (2 Kings 17:9, NAS)
Solomon warned us that sexual sins are usually done in secret. "Passing through the street near her corner; and he takes the way to her house, in the twilight, in the evening, in the middle of the night and in the darkness" (Proverbs 7:8--9, NAS)
Our sins are like the print in a book. It doesn't matter if the book is open or shut--the words are still there.
C. Our sins may only be internal
We are very prone to think in our hearts what we would never consider doing with our hands or saying out loud. "For as he thinks within himself, so he is" (Proverbs 23:7, NAS). It is possible, at least for a time, to keep our sin contained to our minds.
II. Why are Secret Sins so Dangerous?
"Woe to those who deeply hide their plans from the LORD, and whose deeds are done in a dark place, and they say, 'Who sees us?' or 'Who knows us?' " (Isaiah 29:15, NAS)
Let's look at six reasons why secret sins are so dangerous.
A. All sin is hated by God
We tend to think that murder is more of a sin than wanting someone dead.