William Butler Yeats' poem, "The Second Coming", relies heavily on two separate systems of allusion and religious symbolism. The first is Christianity, especial the New Testament Book of Revelation, and the second is Yeats' own personal theological and historical system, which is a combination of Vico, late 19th century spiritualist movements, and the Celtic revival.
The title and main event of the poem refer to the theme of the Second Coming of Christ, which in Christianity is the end of the world and the time at which all people will be judged and sent to either Heaven or Hell. The beast is one of the signs that the second coming is at hand, and its return to Bethlehem is a completion of the historical cycle begun with the incarnation.
Allusion is crucial to the development of the theme of the poem.