Duality is perhaps the major theme of "The Rime of the Ancient Mariner." Duality is the notion that all things have both good and bad within them. Coleridge's poem presents nature and humanity as having dual natures in this way.

Firstly, Coleridge does not present nature in a one-dimensional manner. It is neither a sentimentalized force of goodness nor a hostile presence intent on destroying man. Nature is both beautiful and dangerous, life-giving and untamed. Examine the mariner's description of the weather and the sea after he is cursed:

Water, water, every where,

And all the boards did shrink;

Water, water, every where,

Nor any drop to drink.

The very deep did rot: O Christ!

That ever this should be!

Yea, slimy things did crawl with legs

Upon the slimy sea.

About, about, in reel and rout

The death-fires danced at night;

The water, like a witch's oils,

Burnt green, and blue and white.

Here, the natural world becomes a source of fear. Terms such as "slimy things" and "witch's oil" give the sequence a fearful, gothic tone. The mariner's sin has made it inhospitable and ugly. However, other depictions of nature as something beautiful and lovely are also present in the poem, as when the mariner looks upon the water snakes and is moved to prayer by their beauty.

Coleridge also examines the duality of man. Mankind is capable of great sin and great repentance. The same mariner who kills an albatross without much thought is also capable of blessing beautiful water snakes "unaware," finally allowing love of all living things to expiate him of his sin (hence, the albatross's corpse falling from his neck). There is even the sense that without the mariner's fall from grace and subsequent suffering alone, he would not have been able to know such divine, all-consuming love, giving the poem a sense of what Catholicism calls "felix culpa," or "fortunate fall." Hence, there is a duality even within error, making the moral universe of the poem quite complicated.

One theme of “The Rime of the Ancient Mariner” is the complexity of the natural world. As an arch-Romantic, Coleridge presents the natural world as extraordinarily beautiful and inspiring. But at the same time, he demonstrates that it can also be a source of death and danger, making man vulnerable and reducing him to insignificance within the vast cosmos of he which is but a small part.

When the ancient mariner and his crewmates are drifting along on the sea, they are the victims of the awesome power of nature. The wind has died, the sun beats down fiercely, and the ocean is full of strange creatures. The men are at the mercy of the natural world, and there is absolutely nothing they can do about it.

And yet, even in the midst of this sheer hell, there is beauty. The ancient mariner's eye is caught by some water-snakes swimming in the shadow of the ship. He responds to the extraordinary beauty of their colors, the “Blue, glossy green, and velvet black” in which they are attired. The mariner is so impressed by these remarkable creatures that he lets out a fulsome apostrophe:

O happy living things! no tongue

Their beauty might declare:

A spring of love gushed from my heart,

And I blessed them unaware.

Despite everything, the mariner is still struck by the beauty of the natural world that has brought such unimaginable suffering to himself and his fellow sailors.

When he carelessly kills the albatross, the Mariner sins against God and nature. He sins against nature because God loves all of creation, not just humankind. The poem states:

He prayeth well, who loveth well
Both man and bird and beast.
He prayeth best, who loveth best
All things both great and small;
For the dear God who loveth us,
He made and loveth all.

The theme or underlying meaning of the poem, therefore, is to show how one gets out from under a sin against God. The poem shows that this happens through the redemptive power of the imagination, an important Romantic theme.

The Mariner is guilty of killing the albatross. However, for a long time, he doesn't understand that he has done anything wrong. He can't see the beauty and wonder of God's creation. Therefore, he and his shipmates remain under a curse. It is impossible for the Mariner to achieve forgiveness until he is able to confess he has done something wrong.

Finally, when the Mariner sees and is able to respond to the beauty of the sea snakes, he shows he has developed the imagination and empathy to understand all of creation as blessed. Then the curse is lifted.

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.

Tjhere are a number of possible themes we could point out in this famous poem, but the one that I feel is of particular relevance to today's society, given the environmental disaster that we have made and suffer, is the green message that is clearly present in this poem.

Let us remember that the Mariner is only able to pray again and expiate the curse that lies upon him when he realises fully the consequences of what was for him an unthinking and casual act. Killing the albatross was not done out of malicious intent, yet the Mariner has to accept responsibility for this crime. Part of this process is learning that all creatures in nature are just as special as other creatures. This is why when the Mariner is able to bless the water snakes, the albatross symbolically falls from his neck, as he has learnt this lesson and understands that all of nature must be cherished and possesses its own wonder and beauty.

This is why the final section of the poem in the third from last stanza seems to summarise this theme when it says:

He prayeth best, who loveth best

All things both great and small;

For the dear God who loveth us,

He made and loveth all.

As the Mariner has learnt, it is important for man to live in harmony with all of nature and all of its creatures. This is the only way to ensure that we can live in harmony with God.