The predominant theme in "The Second Coming" is that of a culture or society coming unhinged. Yeats uses apocalyptic language and imagery to suggest that society has reached, or is nearing, the point at which its social and cultural foundations ( reason, Christianity, scientific progress) are eroding underneath it:
The falcon cannot hear the falconer; Things fall apart; the centre cannot hold; Mere anarchy is loosed upon the world...
It is important to realize that Yeats was writing one year after the astonishing carnage of the First World War, in a period of extreme pessimism about Western society. It was easy for intellectuals like Yeats to imagine that society was in peril, indeed it seems a reasonable assessment even nine decades later. As noted above, Yeats deliberately invokes biblical references to the end of the world (the title, after all, is "The Second Coming," but significantly, he does not see this "second coming" as ushering in the reign of God on earth. Rather he seems to imagine that things will be worse, that frightening forces are about the be unleashed. In a final passage, eerily prescient given the events of the Second World War, he wonders:
...what rough beast, its hour come round at last,
Slouches towards Bethlehem to be born?