Title: "The Second Coming" is an allusion to the reappearance of Christ as prophesied in the Book of Revelation.
Lines 4-6: These lines contain two more allusions to the Bible. First, the word "anarchy" calls to mind the reign of Satan on Earth before Christ comes back. However, more specifically, it also brings to mind the Biblical flood that sent Noah packing the wife, kids, and a few pets into the ark. Interestingly, the poem spans the entire length of the Bible in these lines, from Genesis (the flood) to Revelation. Yeats’s image is noticeably more violent than the Bible ("blood-dimmed tide," "drowned"). It’s like the big flood viewed from the perspective of those who didn’t make it into the ark. Another notable thing about these lines is the work being done by the word "loosed," which translates roughly to "unleashed" or "let free." It’s a word that can be applied to a liquid like water, but also has the implication of a more animalistic force. In this way, it prefaces the symbolic unleashing of the "rough beast" later in the poem.
Lines 13-14: The description of the sphinx in the desert recalls several themes from the Bible. First, as we know, the sphinx is that big stone animal that tourists like to snap pictures of in Egypt. In the Book of Exodus in the Old Testament, Egypt is where the Jewish people where held in bondage until they were freed by Charleton Heston, that is, Moses. Also, these lines bring to mind the story of Christ’s temptation by Satan in the desert. So, in a sense, the desert is the devil’s home. Finally, the sphinx itself, as a mash-up of two different animals (man and lion), can be compared with similarly confused species in the Book of Revelation, such as locusts with scorpion tails (ouch!).
Line 19: "Stony sleep," "nightmare," and "rocking cradle" are part of an extended metaphor comparing the "twenty centuries" between Christ and the Second Coming as only one night of an infant’s sleep. The metaphor of sleep suggests either the relative peacefulness or the obliviousness (probably both) which characterized the "twenty centuries" between the First and Second Comings, assuming that the latter is just around the corner.
Line 22: Compared to the weird images inspired by the Book of Revelation, this one’s easy. Christ was born in Bethlehem, so that city is a symbol of the entrance of absolute and messianic forces in the world. In the case of Christ, absolute Good. In the case of the "rough beast," well, let’s just say nobody’s going to be greeting this thing with frankincense and myrrh.