Ecclesiates 12

After focusing and the happenings and things of the this world, Solomon or the Preacher as he calls himself, concludes -

"Let us hear the conclusion of the whole matter: Fear God, and keep his commandments: for this is the whole duty of man.
14 For God shall bring every work into judgment, with every secret thing, whether it be good, or whether it be evil." (Old Testament | Ecclesiastes 12:13 - 14)

That is the conclusion and purpose of our total mortal existence, to fear God and keep His commandments. It is our duty. The Lord said to Moses -

"...I the LORD thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me,
10 And shewing mercy unto thousands of them that love me and keep my commandments." (Old Testament | Deuteronomy 5:9 - 10)

In one sentence, during His mortal ministry, the Savior declared the same -

"If ye love me, keep my commandments." (New Testament | John 14:15)

That is our duty if we desire His mercy.