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.
blow Your trumpets, Angels (lines 1-2)
The speaker sounds like the conductor of a symphony. He places himself in the heart of the action.
For, if above all these, my sins abound, 'Tis late to ask abundance of thy grace, (lines 10-11)
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.
here on this lowly ground, (line 12)
The majesty of the Day of Judgment is placed beside the humbleness of human conditions. The last part of the poem pivots on the consecutive words "there" and "here." The speaker removes himself from the center of the action and places himself on the "ground."
Teach me how to repent; for that's as good As if thou hadst seal'd my pardon, with thy blood. (lines 13-14)
Well, the speaker is still asking for stuff, but at least he wants to know how to "repent," or ask forgiveness for his sins. Repentance is by nature a humble activity.