Q. 45:What did God at first reveal to man for the rule of his obedience?
A. The rule which God at first revealed to man for his obedience, was the moral law.
For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: which shows the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another. Rom 2:14-15