Remember playing tag in school? There was always that kid who, right before you were about to tag him, would cry, "Time out!" and then would make up an excuse: "I have to tie my shoe!" It was a totally absurd excuse, and everyone knew it. For one thing, there are no "time outs" in tag. Professional basketball and football, yes; but tag, no. You either play or you don't play. But this kid made it sound so convincing that you ended up allowing him to take that one time out, on the condition that he wouldn't do it again.
John Donne was that kid. Except he never grew out of his mischievous, "time out"-calling ways. He just got smarter about it; to use the language of his times, he used his "wit" or intelligence.
Donne has graduated from tag to a game with much, much higher stakes: call it "Apocalypse" or "Judgment Day." The way you play this game is to pray for the heavenly powers to separate the sinners from the saved and you hope that your deeds earn you a place among the saved. In this sonnet, Donne starts to play this game but quickly realizes he's in over his head. If there were even the smallest chance of ending up among the damned, you'd want to call the whole thing off, which is exactly what the speaker does.
Donne sounds like the kid playing tag who makes excuses. He's says, "Time out! I forgot the part of the game where I'm supposed to repent my sins. Can you give me a refresher?" OK,
just this once.