He repeats this line four times in the first two stanzas, each for a different sin. But then, in the final stanza, Donne abandons this repetition and the speaker is more sure of his salvation. Why, if he already knows God will forgive him, does the speaker still ask?
When thou hast done, thou hast not done, (5)
It seems like the speaker sees God's forgiveness like an all-you-can-eat buffet (mmm, buffet…); it never runs out. But, if God really is God, wouldn't He already know that? Why would the speaker need to remind Him?
But swear by thyself, that at my death thy Son Shall shine as he shines now, and heretofore; (15-16)
The "Son" Donne is referring to is Jesus, who was the son of God sent to die for mankind's sins. If this bargain still holds up at the speaker's death, he's saying, then he will die forgiven. The speaker is so concerned that this bargain won't hold, though, that he asks God to swear by Himself that it will.