T. S. Eliot's play Murder in the Cathedral reflects his own religious beliefs, which are usually described as Anglo-Catholic. This means that he was a member of the Church of England (Anglican) but of the "high church" party within the church, a group that emphasizes traditional liturgy and hierarchy and is in many ways close to Roman Catholicism without accepting the authority of the Pope. Another major theological element to Eliot's Anglicanism is the notion of salvation by grace through faith, as opposed to salvation by works (and sacraments) as articulated in Roman Catholic theology.
Thus for Eliot, as for most Protestants, salvation is an inward as much as an outward act. Protestants believe that one cannot, in essence, "buy" one's way into Heaven with good works. This theological difference is highlighted in Thomas's response to the fourth tempter:
The last temptation is the greatest treason:
to do the right deed for the wrong reason.
The priests and the chorus act as tempters in three ways, extending the dilemma raised by the fourth tempter of doing good deeds for the wrong reasons. First, by focusing on outcomes of his actions, they try to divert him from acting entirely in accordance with the dictates of his conscience. Second, they talk about uncertainty and doubt, both of which are very much opposed to the certainty Thomas needs to act out of a pure, strong faith. Finally, they offer another type of temptation, which is human sympathy and the desire to stay with and minister to his flock. All of these impulses are really the opposite of what faith demands of Thomas.