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.
This is why Jesus said, "But I say to you that everyone who is angry with his brother shall be guilty before the court; and whoever says to his brother, 'You good-for-nothing,' shall be guilty before the supreme court; and whoever says, 'You fool,' shall be guilty enough to go into the fiery hell. Therefore if you are presenting your offering at the altar, and there remember that your brother has something against you, leave your offering there before the altar and go; first be reconciled to your brother, and then come and present your offering" (Matthew 5:22--24, NAS).
Charles Spurgeon said "A sin is a sin, whether done in private or before the wide world...Do not measure sin by what other people say of it. Measure sin by what God says of it..."
B. God especially hates hypocrisy
It would be better for a person to say they want to follow Satan do what pleases him, than to say they want to follow God and then do what pleases Satan. It must be particularly repulsive to God when someone claims to love God publicly, but then disrespects Him to His face in private. "They profess to know God, but by their deeds they deny Him, being detestable and disobedient and worthless for any good deed" (Titus 1:16, NAS)
Hypocrisy stirs up the anger of God. "Beware of the scribes, who like to walk around in long robes, and love respectful greetings in the market places, and chief seats in the synagogues and places of honor at banquets, who devour widows' houses, and for appearance's sake offer long prayers. These will receive greater condemnation" (Luke 20:46--47, NAS).