The two interconnected themes of Donne's "Meditations XVII" (1624) are: 1. All men are mortal 2.The lives of all men are interrelated and interconnected.
Donne is seriously ill and he fears that he might die soon which in turn leads him to pen his thoughts about life and death in the Christian context.
At the time of the Renaissance, life expectancy was low mainly because of the plague epidemic. The funeral bell which was rung by the local church to announce the death of a person could be heard ringing often at the time of the plague epidemic and those alive would wonder as to the identity of the person who had just died; they might even feel proud that they have escaped death but this line warns them that one should not enquire as to who has just died but must be warned that he himself could be the next person to die:"this bell calls us all."
This is because one God is the Creator of all mankind and at the time of the Renaissance all belonged to one Universal Church:"all that she (the Church) does belongs to all." So Donne insists that when we hear the funeral bell announcing the death of a person we should be aware of its real significance: it is not merely announcing the death of a stranger but is actually announcing our own death "passing a piece of himself out of this world......Any man's death diminishes me, because I am involved in mankind....it tolls for thee."
Donne underscores the theme of his "Meditation" that all human lives are interconnected by the use of a metaphor or conceit. The metaphor that he uses is that of an island. He asserts that,
No man is an island, entire of itself; every man is a piece of the continent, a part of the main. If a clod be washed away by the sea, Europe is the less.
meaning that all humanity is one large unit created by God- "all mankind is of one author" - and that one person's death affects all humanity. When a person dies it is a loss for all the others. No human being can live or survive in isolation.
Europe is a huge continent. Donne insists that even if a 'clod' - a small piece of land - is washed away by the sea the whole continent is affected. Here the continent of 'Europe' represents the whole of humanity and 'the clod' represents one human being.
Just like how the huge continent Europe is affected by a small bit of land being washed away by the sea, so all humanity is affected by the death of an individual.