If there is no God then, it does not matter how I live. My life, any life is irrelevant in the scheme of a vast universe that takes no notice of my coming and passing. However, if there is a God then, my relevance is entirely in His hands. Those who fulfill their purpose are relevant. The rest are no more than cosmic dust, they come and go and make no impact of value in Creation.
The shortened version of the Westminster Catechism comes to mind. The chief end of man is to glorify God and enjoy Him eternally.
"To the Jews who had believed him, Jesus said, 'If you hold to my teaching, you are really my disciples. Then you will know the truth, and the truth will set you free.'" John 8:31-32