What prompted Abel to bring the required sacrifice?
Abel was prompted by his conscience. God placed conscience in Adam to precipitate confession, and Abel's conscience convicted him, so he listened to it and brought the required sacrifice. Scripture describes conscience as an inward witness, with thoughts accusing or excusing, which explains the inward conviction that moved Abel.
Les teaches that conscience was to precipitate confession, and that is what prompted Abel to bring the required sacrifice. His conscience convicted him, and he listened to it. Les contrasts this with the human capacity to spurn conscience—to burn it until it is no longer active—yet it remains and cannot be ignored. The surrounding context describes Gentiles who have not the Law doing by nature the things contained in the Law, showing the work of the Law written in their heart, their conscience bearing witness, and their thoughts accusing or excusing. That, Les says, is human nature.