Q: What is the duty which God requireth of man?
A: The duty which God requireth of man, is obedience to his revealed will.

Deuteronomy 29:29. The secret things belong unto the LORD our God: but those things which are revealed belong unto us and to our children for ever, that we may do all the words of this law.
Micah 6:8. He hath shewed thee, O man, what is good; and what doth the LORD require of thee, but to do justly, and to love mercy, and to walk humbly with thy God?
1 Samuel 15:22. And Samuel said, Hath the LORD as great delight in burnt offerings and sacrifices, as in obeying the voice of the LORD? Behold, to obey is better than sacrifice, and to hearken than the fat of rams.

Westminster Project - See more on question 39 from Francis Beattie on the Law of God and Christian Liberty