Manís duty to God
Man was created by God as the crown of all his creation, in God's image and after His likeness (holy, righteous and good), with a reasonable soul, to rule on God's behalf over all creation.
Man forfeited this high privilege on account of his sin and the breaking of the covenant of works. Thus man became blind and ignorant; God had to speak to him in order to instruct him and bring him back to a right knowledge of God and his will.
The Bible therefore is a redemptive message: it shows man his duty to God, now that he is a sinner in need of redemption.
Naturally, the first and primary duty is for man to repent, to turn to God from idols to serve the living and true God (1 Thessalonians. 1:9). God commands all men everywhere to repent (Acts 17); a repentance that is genuine and credible by having good works. "Bring forth fruit in keeping with your repentance." Repentance is man's duty; he is held responsible to repent, though in his fallen state, he cannot repent or convert himself unless God convert him (Jeremiah 31:18; Lamentations 5:21).
Joined with repentance and inseparable from it is faith, without which it is impossible to please God (Hebrews 11:6). "This is his commandment, that we believe in the Name of the Son of God, and love one another, as he gave us commandment" (1 John). Likewise faith is a gift from God, instilled in us by exercise of the Holy Spirit. Where there's faith and repentance a man is in a right relationship with his Maker. He will realise that his duty is to do everything in the name of the Lord Jesus Christ (Colossians 3:17), even though it be such "trivial" things as eating and drinking (1 Corinthians 10:31).
Man was created for God's glory, that God might manifest himself and be magnified in, through, and above his creature.
Augustine's sentiment, "Thou has made us for thyself and our hearts are restless unless they find rest in thee," though not a quote from Scripture, is wholly scriptural (cf. Matthew 11:28).
Solomon's search for meaning in life came to this all-embracing conclusion: "Let us hear the conclusion of the whole matter: Fear God, and keep his commandments: for this is the whole duty of man" (Ecclesiastes 12:13).
To wrap up man's duty to God, I find no better sentence, more comprehensive and far-reaching, than Paul's statement in Romans 11:36 "For of him, and through him, and to him, are all things: to whom be glory for ever. Amen."