Charles H. Spurgeon on Psalms 51:6

Behold. Here is the great matter for consideration. God desires not merely outward virtue, but inward purity, and the penitent's sense of sin is greatly deepened as with astonishment he discovers this truth, and how far he is from satisfying the divine demand. The second "Behold" is fitly set over against the first; how great the gulf which yawns between them! Thou desirest truth in the inward parts. Reality, sincerity, true holiness, heart fidelity, these are the demands of God. He cares not for the pretence of purity, he looks to the mind, heart, and soul. Always has the Holy One of Israel estimated men by their inner nature, and not by their outward professions; to him the inward is as visible as the outward, and he rightly judges that the essential character of an action lies in the motive of him who works it. And in the hidden parts thou shalt make me to know wisdom. The penitent feels that God is teaching him truth concerning his nature, which he had not before perceived. The love of the heart, the mystery of its fall, and the way of its purification —this hidden wisdom we must all attain; and it is a great blessing to be able to believe that the Lord will "make us to know it." No one can teach our innermost nature but the Lord, but he can instruct us to profit. The Holy Spirit can write the law on our heart, and that is the sum of practical wisdom. He can put the fear of the Lord within, and that is the beginning of wisdom. He can reveal Christ in us, and he is essential wisdom. Such poor, foolish, disarranged souls as ours, shall yet be ordered aright, and truth and wisdom shall reign within us.