Charles H. Spurgeon on Psalms 139:23

Search me, O God, and know my heart. David is no accomplice with traitors. He has disowned them in set form, and now he appeals to God that he does not harbour a trace of fellowship with them. He will have God himself search him, and search him thoroughly, till every point of his being is known, and read, and understood; for he is sure that even by such an investigation there will be found in him no complicity with wicked men. He challenges the fullest investigation, the innermost search: he had need be a true man who can put himself deliberately into such a crucible. Yet we may each one desire such searching; for it would be a terrible calamity to us for sin to remain in our hearts unknown and undiscovered. Try me, and know my thoughts. Exercise any and every test upon me. By fire and by water let me be examined. Read not alone the desires of my heart, but the fugitive thoughts of my head. Know with all penetrating knowledge all that is or has been in the chambers of my mind. What a mercy that there is one being who can know us to perfection! He is intimately at home with us. He is graciously inclined towards us, and is willing to bend his omniscience to serve the end of our sanctification. Let us pray as David did, and let us be as honest as he. We cannot hide our sin: salvation lies the other way, in a plain discovery of evil, and an effectual severance from it.