The Question and Answer section for John Donne: Poems is a great resource to ask questions, find answers, and discuss the novel.
I'm sorry, this is a short-answer forum. You'll need to ask your questions separately.
As for the first quote, Donne is essentially saying that for every new philosophy we accept, prior beliefs must be questioned.
The main theme in For Whom the Bell Tolls is death.
You can find the poem on-line. The summary and analysis can be found at the GradeSaver link below: