The quote that is referred to in your question is one that is taken from the first stanza of the poem and serves to describe the church that the speaker has just entered. Its importance lies in what it reveals about the speaker from what is said and the way that it is said. You might like to think about the phrase "Some brass and stuff/ Up at the holy end." The fact that the speaker only recognises the metal brass and then adds to the description by using the word "stuff" clearly points out the speaker's lack of technical knowledge about churches and architecture. As he says later on about another aspect of church architecture, "Somebody would know; I don't." This is somebody who does not get bogged down in the details in his attraction to churches.
Also, note the vagueness of the phrase "the holy end." The speaker knows enough to know that the church should have a "holy end" in its architecture, but beyond that his knowledge is limited. He does not know the correct term for this and the phrase "holy end" seems to have a rather mocking tone as if the speaker is making fun of Christianity and the importance of beliefs that have sustained churches for so long.
The importance of this introductory phrase taken from the first stanza therefore lies in the way that it reveals key facts about the speaker's stance towards church, that further complicates his character when we consider the rather paradoxical relationship that he has with all things religious.