The Sublime is the emotion of awe and terror we experience when we gaze on nature and feel both overwhelmed by its beauty but at the same time frightened and humbled by its grandeur and power. The Romantics were particularly drawn to the Sublime, for they believed it revealed God's presence in the world.
A typical Sublime location was a snowy, misty mountaintop, where a person stands on the edge of a cliff or boulder looking down into a rocky gorge or at a valley far below. The scene is both beautiful and terrifying at the same time, impressing on the onlooker the enormous power of God, who creates both loveliness and frightening magnificence. We are small against God in such a landscape.
The Mariner's problem as the poem opens, as he sails to uncharted regions of the Antarctic that have never been explored, is his inability to feel the Sublime despite the immense beauty and frightening grandeur of his setting. We get a sense of the Sublime in the following passage:
And now there came both mist and snow,
And it grew wondrous cold:
And ice, mast-high, came floating by,
As green as emerald.
An iceberg as tall as the ship and green as an emerald floats by, but the mariner is unimpressed by its magnificence and does not see God in it. He is so blase and lacking in awe that he even kills the albatross, a messenger of God's grace, just because he can.
The Sublime moment comes when finally, after understanding nature only as something "painted," something apart from him, something that cannot slake his spiritual thirst, that does not give him one "drop" to drink, the mariner finally has his moment of deep emotional response to the sea snakes:
Within the shadow of the ship
I watched their rich attire:
Blue, glossy green, and velvet black,
They coiled and swam; and every track
Was a flash of golden fire.
He can actually feel the Sublime in the awe he experiences as he watches the beauty and the power of the snakes. They are not "painted" and static but alive and full of fire. The Mariner thinks:
O happy living things! no tongue
Their beauty might declare:
A spring of love gushed from my heart,
And I blessed them unaware
This Sublime moment, the ability to feel overwhelmed and filled with love—to perceive the divine in nature—releases the mariner from his curse.
To reinforce that experiencing the Sublime leads us closer to the divine source, we are told that:
He prayeth best, who loveth best
All things both great and small;
For the dear God who loveth us,
He made and loveth all.