"Batter my heart, three personed God"/ similie

"Batter my heart, three personed God" by John Donne is a very interesting poem!  I am not exactly sure what it is about, but like always I have an idea.  I am taking this poem literally, and I think it is really about God.  "As yet but knock, breathe, shine, and seek to mend."  This is saying that God always knocks, and wants to be a part of our life.  However do we always listen?  "That I may rise and stand."  This means that when God calls, we should rise and stand before Him.  "I, like an usurped town, to another due."  Truthfully, I just know this is an example of a simile, and I am not sure what it is about.  I took from this poem that the speaker is saying that we are all sinners.  He is asking God to cleans us and accept us even though we have done wrong.  I know that God always forgives us.