The Rime of the Ancient Mariner is Exhibit A for evidence of Coleridge's wild imagination, which was helped along by a moderate-to-heavy opium usage. He takes bits and pieces of mythology and symbolism from Greek and Roman myth and Christian scripture and manufactures a modern ghost-and-zombie story complete with visits from Death and his grisly accomplice, Life-and-Death. The power of supernatural forces over the ship and its crew helps to make the Mariner's own feebleness clear. The supernatural is often related to meteorological (weather) and astrological events in this poem.
The moon represents the Virgin Mary, while the force "nine-fathoms" under the sea represents a Greek god like Poseidon or Hades. Christian figures have control over pagan ones in this poem, but both coexist.
Although the Mariner considers himself a Christian, The Rime of the Ancient Mariner is not a Christian poem.