Q: 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.

Romans 2:14-15. 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 shew 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.)
Romans 10:5. For Moses describeth the righteousness which is of the law, That the man which doeth those things shall live by them.

Westminster Project - See Thomas Vincent's as he explains and proves the WSC from Scripture