There are no small sins. Either we repent thoroughly or they grow and ultimately dominate our lives or the lives of our children. 2 Kings 3:2-5 describes the sons born to David while king at Hebron. "Third, Absalom the son of Maacah, the daughter of Talmai king of Geshur."
Despite clear biblical prohibition against marriages to unbelievers, David married the daughter of a foreign king. It was a small thing, you say. To the watching world it was small. David had many wives, they were all sons of Abraham except Maacah, but no sin is small to God, and God was watching. Sin has consequences, and they seldom appear immediately.
Thumb forward to 1 Kings 11. A couple of decades have passed. David is dead, and his son, Solomon, reigns in his place. Solomon is now eating from the tree his father planted. Solomon watched his father. There was no outward repentance about his marriage to Maacah. Solomon imitates his father. David's discreet sin now matures into full-blown, life-wrecking, culture-seducing idolatry.
"Now King Solomon loved many foreign women, along with the daughter of Pharaoh: Moabite, Ammonite, Edomite, Sidonian, and Hittite women, from the nations concerning which the Lord had said to the people of Israel, “You shall not enter into marriage with them, neither shall they with you, for surely they will turn away your heart after their gods.” Solomon clung to these in love. He had 700 wives, princesses, and 300 concubines. And his wives turned away his heart. For when Solomon was old his wives turned away his heart after other gods, and his heart was not wholly true to the Lord his God, as was the heart of David his father. For Solomon went after Ashtoreth the goddess of the Sidonians, and after Milcom the abomination of the Ammonites. So Solomon did what was evil in the sight of the Lord and did not wholly follow the Lord, as David his father had done. Then Solomon built a high place for Chemosh the abomination of Moab, and for Molech the abomination of the Ammonites, on the mountain east of Jerusalem. And so he did for all his foreign wives, who made offerings and sacrificed to their gods." (1 Kings 11:1-8).
The consequences of David's little compromise was massive. The "high places" to these horrible idols were not removed until the reign of King Josiah, almost 3 centuries later.
What small unconfessed sin lingers in our lives? What small compromises have captured our hearts? "Do not be deceived: God is not mocked, for whatever one sows, that will he also reap. For the one who sows to his own flesh will from the flesh reap corruption, but the one who sows to the Spirit will from the Spirit reap eternal life." (Galatians 6:7-8).