Literary devices in "The Second Coming" include the following:
antithesis: Antithesis is when two contrasting ideas or images are put together. A central motif of this poem is that the second coming Yeats envisions contrasts sharply with the second coming promised in the Bible. This is not Jesus the savior returning to earth to establish the kingdom of God. It is the exact opposite, a second coming of a primal evil loosed upon the world. Imagining the pitiless beast coming to Bethlehem to be born is also antithetical to the idea of that location as the birthplace of Jesus, who is understood as the prince of mercy and peace.
aphorism: An aphorism is a short, pithy statement of universal truth. When Yeats writes
The best lack all conviction, while the worst
Are full of passionate intensity
he is communicating a central truth about his time.
Yeats leans heavily into imagery in this poem, as most poets do. Images are words that convey what we can experience with the five senses: sight, sound, taste, smell, and touch. Yeats uses the image of a gyre turning round and round in the first stanza. He focuses on visual imagery in the second stanza, as he paints a picture of a beast with the body of a lion and the head of a man moving slowly towards Bethlehem.
repetition: While the poem doesn't use a conventional rhyme scheme, Yeats builds a sense of rhythm, especially in the beginning of the poem, by repeating words such as "turning," "surely," and "Second Coming."
simile: Simile is a comparison using like or as. Yeats writes that the beast has "a gaze blank and pitiless as the sun," which conveys the sense of a blinding, relentless stare.
understatement: When Yeats writes "mere anarchy" is loosed, the word "mere" devalues something important. Loosing anarchy on the world is not inconsequential, so the statement functions as a form of irony by saying the opposite of what is meant. Yeats wants to jolt us into the understanding that this anarchy is important and is not something to shrug off.