Q. 46. 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 what the law requires, they are a law to themselves, even though they do not have the law. They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or even excuse them Romans 2:14-15
for sin indeed was in the world before the law was given, but sin is not counted where there is no law. Yet death reigned from Adam to Moses, even over those whose sinning was not like the transgression of Adam, who was a type of the one who was to come. Romans 5:13-14