Our first duty is to satisfy the spiritual master, who can arrange for the Lord's mercy. A common man must first begin to serve the spiritual master or the devotee. Then, through the mercy of the devotee, the Lord will be satisfied. Unless one receives the dust of a devotee's lotus feet on one's head, there is no possibility of advancement. Unless one approaches a pure devotee, he cannot understand the Supreme Personality of Godhead. — Sri Chaitanya caritamrita, Madhya Lila, 11.58.
It is our duty to act in Krishna consciousness because we are constitutionally parts and parcels of the Supreme.
Path of Perfection, Chapter 1
The occupational activities a man performs according to his own position are only useless labor if they do not provide attraction for the message of the Personality of Godhead.
One who performs his duty without attachment, surrendering the results unto the Supreme Lord, is unaffected by sinful action, as the lotus leaf is untouched by water.