The poem that begins with "No man is an island" is Meditation 17 from "Devotions Upon Emergent Occasions," prayers written by John Donne during a difficult time in his life. The words of this meditation were originally written in prose rather than poetry.
The theme of this work revolves around man's involvement with the world around him. He is not a separate entity as much as he is a small part of the whole, and the world is not the same without each part of that whole. Donne states that the death of any person diminishes everyone else. The meditation ends by advising that no one need ask "for whom the bell tolls" because in a sense the death of one is a death to some degree of all.