T.S. Eliot's Murder in the Cathedral was commissioned by Bishop George Bell as a response to Nazi abuse of power. It calls on people to resist tyranny and is based on the real story of Thomas Becket, the Archbishop of Canterbury who was murdered for opposing King Henry II.
In the play, the Becket character realizes that he will have to oppose the king, and faces three temptations to avoid doing this, temptations based on the gospel account of the devil's temptations of Christ in the wilderness. However, Eliot burdens Becket with a fourth temptation: the desire to become a martyr for worldly reasons rather than spiritual ones. Becket struggles with whether he wants to be a martyr for ego gratification or to serve the will of God.
As Becket comes to understand, martyrdom is "never the design of man," for "the true martyr is he who has become the instrument of God" and "who no longer desires anything for himself."
Martyrdom is central to this play both because Becket becomes a martyr, dying to uphold his faith and because the play suggests that principled people must oppose tyranny, but also because the play calls into question what motivates martyrdom, arguing that it should be done for spiritual reasons, not self-glorification.