| Quote #1
round earth's imagined corners (line 1)
Right away the speaker demonstrates his appreciation for science and intelligence by contrasting the modern view that the earth is round with the view from the Book of Revelation that it is flat. You don't have to view this as an aggressive stance for him to take. The word "imagined" acknowledges that the idea of a world with corners has value as a poetic or creative truth.
| Quote #2
The speaker sounds like the conductor of a symphony. He places himself in the heart of the action.
| Quote #3
For, if above all these, my sins abound,
All of a sudden, the speaker turns humble. His sins might be even worse than those of the all the souls that have died. His sins are everywhere – they "abound." Still, he does admit that he wants to God to give him grace, so he's still far from passive.