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 Gentiles, who do not have the law, by nature do the things in the law, these, although not having the law, are a law to themselves, who show the work of the law written in their hearts, their conscience also bearing witness, and between themselves their thoughts accusing or else excusing them. Romans 2:14-15