Dialogue, of course, contributes to the realistic impact of any piece of fiction, even a very brief story such as "Salvation." Here, the voices of both the clergyman and Hughes's aunt emphasize how strong their wish is that Langston get up and acknowledge or accept salvation, as he's expected to do. Without these breakouts, so to speak, into the speaking of the characters in the story, we would have a less direct awareness of the emotion conveyed in the scene.
Westley's voice has a more specific function, however. Two things indicate that Westley doesn't genuinely "believe": his "taking the Lord's name in vain" and his telling Langston that they should stand up and go to the front only so that the service will be over and done with and they can go home.
By extension, Hughes is implying that under pressure from others, people agree to what is expected of them. Or is it that all of the participants are thinking the same thing as Westley—that the whole ceremony is a waste of time—and are just pretending to be "saved," as Langston himself does in the end? But we don't know for sure what Langston himself actually believes. At the conclusion he tells us that he wept because "Jesus didn't come to help me." But does he believe Jesus did appear to at least some of the others? Readers must decide this question for themselves.