The [Stoic] concept of [natural law] also implies a deistic conception of God. It thinks of God as having written certain ethical principles on the human heart, at the creation. Even when he is out of fellowship with God he has these principles in him and can direct his life accordingly. But Paul believes in God as living and ever active with men, even with the heathen, in life’s concrete situations, showing them what is good and what is required of him… He has written "the works of the law" in their hearts so that, if they do otherwise in the concrete situation, they are aware they have done evil.
 

Anders Nygren. Commentary on Romans. New edition. Augsburg Fortress Pub, 1978. On Romans 2:1-3:20 (2).