What did God at first reveal to man for the rule of his obedience?
The rule which God at first revealed to man for his obedience, was the Moral Law.
Romans 2:14-16 (NASB)
14 For when Gentiles who do not have the Law do instinctively the things of the Law, these, not having the Law, are a law to themselves, 15 in that they show the work of the Law written in their hearts, their conscience bearing witness and their thoughts alternately accusing or else defending them, 16 on the day when, according to my gospel, God will judge the secrets of men through Christ Jesus.
Everyone has a consciences that tells them what is right or wrong; however, we are easily deceived and even deceive ourselves; therefore, until our inner most desire is changed by God, man needs an external law to tell what is right and wrong.