The spirit of man is the lamp of the Lord,
searching all his innermost parts.
This spirit is our conscience which bears witness that the law of God is written on our hearts (Romans 2:15). God has placed within all of us - those inhabited by God's Spirit and those who are not - his inner moral compass, or, as this proverb notes, his inner moral lamp that searches our thoughts and actions. This is why everyone is without excuse in their rejection of God's law. We all know better, however much we may protest to the contrary.
Such a truth puts into perspective those who are most "moral." Their very interest in doing what is right betrays the fact that they know there is a God who requires righteousness; and yet they cannot bring themselves to acknowledge his existence; or if they believe he exists, then to acknowledge his rights over them; or if they believe he has such rights, then to believe in him as he demands - through Jesus Christ; or if they profess to believe in Christ, they nevertheless do not acknowledge his true work on the cross - to save sinners like them who cannot save themselves.
And then there are those of us who profess all these things about God and about Christ; who acknowledge our dependence upon our Lord and love for him; who profess obedience to him, but will not expose our darkest corners to God's lamp. We fear too much what we will see. And so we will look only at the sin we can handle and congratulate ourselves for being severe with our sin.
Use the lamp for its purpose - to fully examine yourself. And use your inner lamp with that greater lamp - the Word of God - so that you might examine all your innermost parts with the clearest light. Are you afraid of what you will see? You will see nothing that God does not know already exists. He sees you completely and loves you completely. Your spirit will find nothing that his Spirit has not already exposed. And that same Spirit is already at work within you to bring cleansing and light and hope.