I disagree partly with lit24. While I believe he is very right in saying one of Donne’s themes was the interconnected human race, I believe he was less concerned about turning their attention from themselves. He was more saying that since all of the church is connected, we should not ask who the bell tolls for, we should feel pain for one a piece of the church that is now closer to God.
The two interconnected themes of Donne's "Meditations XVII" (1624) are: 1. All men are mortal 2.The lives of all men are interrelated.
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."