Sin and repentance are the central themes of "The Rime of the Ancient Mariner." The Mariner commits a terrible sin when he kills the albatross, one of God's beloved creatures. He spends the rest of his life trying to atone for his sin through his suffering and humility. He devotes his life to warning people about the dangers of sin, using his own life as a cautionary tale.
The overt story of the poem is the rather terrifying tale of a sailor beset by troubles at sea. The underlying themes and the real subjects of the poem are christianity and the supernatural. The mariner, who evetually becomes "ancient," decides on a whim to shoot the albatross that is following the boat. He knows this is apparently considered bad luck, but what can be the harm.
He eventually pays the price for his foolishness and is redeemed as he learns the beauty of the life of the sea and the animals within and above it and spends the remainder of his life telling others his cautionary tale.
The theme of the poem is:
'If you will harm nature, Nature will take its revenge'