At the round earth's imagined corners (Holy Sonnet 7) Religion Quotes

The speaker almost goes out of his way to refer to the modern scientific knowledge that the earth is round, in contradiction to the passage from Revelation 7: "After this I saw four angels standing at the four corners of the earth…"

You've heard of a mash-up, right? You know, like when a DJ mixes The Beatles' White Album with Jay Z's Black Album to create a Grey Album? In the first two lines, Donne has created a miniature mash-up with two passages of the Book of Revelation. You have the four angels standing at the corners of the earth (Revelation 7), and then the seven angels who blow their trumpets to set off scenes of Apocalyptic violence (Revelation 8).

The "flood" refers to the Biblical flood in the Book of Genesis, from which only Noah and his family were spared. Most scholars think that the "fire" alludes to the fires of the Apocalypse, which, according to the Book of Revelation, burn up a third of the earth, a third of the trees, and a third of the green grass (Revelation 8:7).