The Third and Fourth Heads of Doctrine: Human Corruption, Conversion to God, and the Way It Occurs
Article 4: The Inadequacy of the Light of Nature
There is, to be sure, a certain light of nature remaining in man after the fall, by virtue of which he retains some notions about God, natural things, and the difference between what is moral and immoral, and demonstrates a certain eagerness for virtue and for good outward behavior. But this light of nature is far from enabling man to come to a saving knowledge of God and conversion to him — so far, in fact, that man does not use it rightly even in matters of nature and society. Instead, in various ways he completely distorts this light, whatever its precise character, and suppresses it in unrighteousness. In doing so he renders himself without excuse before God.
Article 5: The Inadequacy of the Law
In this respect, what is true of the light of nature is true also of the Ten Commandments given by God through Moses specifically to the Jews. For man cannot obtain saving grace through the Decalogue, because, although it does expose the magnitude of his sin and increasingly convict him of his guilt, yet it does not offer a remedy or enable him to escape from his misery, and, indeed, weakened as it is by the flesh, leaves the offender under the curse.
Daily Confession 