Natural theology is knowledge of God we get from creation, not a supernatural revelation through prophecy, visions or Scripture. It leaves us without excuse and guilty before God, but cannot bring us saving knowledge of Christ. This means Adam had a moral sense of gods will before the fall. He wasn't naive and clueless, as we often think of him and Eve. Did they really have NO knowledge of good and evil until they ate the fruit?
Some puritans asserted that there was no supernatural knowledge of God before the fall, to keep a strong distinction between the covenant of works and the covenant of grace, but this was a minority view. It's hard to deny when we read that Adam walked with God in the cool of the day (Gen 3:8). Thomas Goodwin said Adam's faith had no supernatural knowledge of God at all, and that no grace existed before the fall. The authors admit this as a minority view, too, but give it lots of attention. It's true Adam wouldn't have trusted God the same way after the fall as before it. But Adam certainly enjoyed God's unmerited favor before he sinned.
This knowledge of God comes from within ourselves, God having implanted it in us, and from the world around us.