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.

In Murder in the Cathedral, why do the Priests and Chorus tempt Thomas before the Interlude?

Before the Interlude, and after the four Tempters have completed their various attempts to encourage Thomas to swerve from his duty to the church and his faith, the Chorus, the three Priests and the Tempters all join in together in a rhythmic series of statements designed to express their fears and doubts about the resolution of Thomas to stay in Canterbury and not to give in to the King's will. These different characters combine to create a real atmosphere of terror and foreboding as both Thomas and the audience become aware of the significance of the actions of Thomas and the likely consequence. Note what these characters say:

C: A man may walk with a lamp at night, and yet be drowned in a ditch.
P: A man may climb the stair in the day, and slip on a broken step.

Each of these examples relate to the situation of Thomas. Even if Thomas makes his decision to stick to his principles with the best intentions and in the full light of understanding, disaster can still happen, as is the case with the two situations expressed by the Chorus and the Priests. This episode in the play therefore represents a warning to Thomas about the consequences of his actions and the likely outcome.