I believe you are referring to Proverbs 20:27 which says, “The spirit of man is the candle of the LORD, searching all the inward parts of the belly.”  Chapter 20 of Proverbs is full of proverbs of warning and instructions about personal conduct. Each verse in this chapter is a different proverb.

In verse 27, the word ‘spirit’ is literally ‘breath’.  It is the same word as we read in Genesis 2:7, “And the LORD God formed man of the dust of the ground, and breathed into his nostrils the BREATH of life; and man became a living soul.”  This ‘breath of the Lord’ made Adam into a moral being, capable of understanding, choosing, and to be responsible for his actions. The breath of the Lord kindled a light in man.  It is here called “the candle of the Lord.” It is because of the sin of Adam, which brought about his fall that man has a conscience and conscience has a place of pre-eminent importance in our nature.  God has given to every man a mind, which he enlightens by his own Spirit, that the man knows how to distinguish good from evil; and conscience, which springs from this, searches the innermost depths of the soul.

It is the conscience that causes us to approve or reprove our actions and thoughts.  There is a conscience in every person, which, unless seared, discerns what is in man, considers his actions, and either excuses or accuses each act.  Of course, there are those who through constant refusal of the truth lose the ability to discern right from wrong.  We read in 1 Timothy 4:1-2, “Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils; Speaking lies in hypocrisy; having their conscience seared with a hot iron.”  This is in reference to the process of cauterization where, using heat, a part of the body is rendered rigid and hard, and is dead to sensibility.  So, with the unbeliever who rejects the truth, but gives way to evil spirits and doctrines.  This ‘candle of the Lord’ can be diminished to the point that a person can become reprobate, without a sensitive conscience.

Man is duly warned in Romans 1:28-32, “And even as they did not like to retain God in their knowledge, God gave them over to a reprobate mind, to do those things which are not convenient; Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers, Backbiters, haters of God, despiteful, proud, boasters, inventors of evil things, disobedient to parents, Without understanding, covenantbreakers, without natural affection, implacable, unmerciful: Who knowing the judgment of God, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them.” 

The ‘candle of the Lord’ is a moral sense that is a direct gift from God which enables us to see our real condition.  We read in Job 32:8, “But there is a spirit in man: and the inspiration of the Almighty giveth them understanding.”  As believers on the Lord Jesus Christ, may we be constantly before the Lord, asking Him to give us that understanding and to keep our conscience sensitive to His truth, that we might follow Him.  David prayed in Psalm 139:23-24, “Search me, O God, and know my heart: try me, and know my thoughts: And see if there be any wicked way in me, and lead me in the way everlasting.”  May that be our constant prayer as well.  (374.4)