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…"
blow Your trumpets, Angels (lines 1-2)
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).
All whom the flood did, and fire shall o'erthrow (line 5)
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).
For, if above all these, my sins abound, 'Tis late to ask abundance of thy grace, (lines 10-11)
In Christian thought, God's grace is limitless. Because, grace refers to God's capacity to forgive people who are not worthy of forgiveness, anyone who has ever lived or who will ever live is included in this category, so God's capacity to forgive must be infinite.
Teach me how to repent; for that's as good As if thou hadst seal'd my pardon, with thy blood. (lines 13-14)
The speaker shows how well he has learned his Sunday school lessons, even as he asks for God to teach him. According to the New Testament, God did "pardon" humanity, in a way, when he sent his son Jesus to die for everyone's sins. Jesus' blood is also God's.