'The Lord looks at the heart.' 1 Samuel 16:7 NLT
Pastor and author John MacArthur says:
'Jesus' exposition of the law is a devastating blow against the lie that image is everything. Secret sin is especially abhorrent because: (1) God sees the heart. In fact, if we realised he's the only audience we'd be less inclined to write it off. It's folly to mitigate sin by keeping it private... it's double-folly to think you're better than others because you sin privately... and it's the height of folly to conceal it. "He who covers his sins will not prosper" (Proverbs 28:13 NKJV). (2) Sinful thoughts originate from the same source as sinful deeds. When Jesus said hatred carries the same guilt as murder, and that lust is essentially adultery, he wasn't saying there's no difference in degree... he was saying that a lustful person has no right to feel superior to a fornicator. The fact that somebody thinks such thoughts proves they're capable of immoral acts, and someone who hates his brother already has murder lurking in his heart. (3) Hypocrisy compounds hidden sin. Why? Because it means covering it up. Jesus called hypocrisy "the leaven of the Pharisees" (Luke 12:1 NKJV) because it compounds itself like leaven. It sears your conscience and paves the way for other character-damaging sins... When somebody tries to tell you appearances are everything - don't buy it! Your secret life is a litmus test of your character: "As he thinks within himself, so he is" (Proverbs 23:7 NASB). If you want to know who you really are, look at your private life. Then gaze into the mirror of God's Word and let him disclose and correct the thoughts and intents of your heart.'
Soulfood: Ecc 10-12, John 10:34-42, Ps 102:12-17, Prov 30:21-23