The entire play is built around the inevitable death of Thomas as he chooses to be obedient to God and God's call on his life rather than give in to the king's commands, which Thomas views as being against God's calling on him. The central theme is therefore that of obedience, as Thomas shows himself to be utterly obedient, even to the point of dying. This is shown most clearly in one of his last speeches where he remonstrates with the priests to open the door and let the knights, who he knows will kill him, into the cathedral. Note what he says:
For every life and every act
Consequence of good and evil can be shown.
And as in time results of many deeds are blended
So good and evil in the end become confounded.
Thomas willingly gives his life to his God, trusting that the consequences of his actions and those of his king in killing him will eventually, in God's perfect timing, become shown for what they are. Thomas eventually is strong enough in himself to resist all temptation offered in the form of the Tempters and is then free to face his destiny, remaining free to die with his loyalty to God in tact. In the major conflict in the play, which is between spiritual beliefs and earthly temptations, Thomas shows that he firmly conquers earthly temptations through his radical obedience.