"At the round earth's imagined corners" is a fantasy of the Last Judgment. In Christian theology, the world as we know it has a definite end point: the Apocalypse. All the people who have died throughout time must wait until the Apocalypse to receive their final judgment. It is at this point when the good are separated from the bad. Christianity teaches that a person can always ask God for forgiveness, but once the Last Judgment comes, the opportunity is lost. The speaker calls off his fantasy when he begins to worry that maybe he hasn't repented enough.
Donne describes three classes of people in this poem: the obviously damned, the obviously saved, and people, like himself, who are need of repentance in order to be saved, but who do not fall into one of the obvious cases.
The speaker's admission that his own sins might be the greatest of all takes advantage of a classic Christian trope in which one shows humility by pretending to be the lowest of the low.