The central idea of "Church Going" by Philip Larkin is that while religion is now no more than an antiquated superstition, there will always be a need for some people to search for answers in religion to justify their existence.
This is clear because the visit to the church prompts the speaker to question why he stops at such places as he often does. The description of the church suggests he is not a church goer. This is clear when he refers to the "brass and stuff" at the front of the church. The speaker then goes to the front, reads aloud from the Bible, and then leaves a donation stating the visit "wasn't worth stopping for." The speaker then asks why he does often stop at churches such as this one, setting up the central idea of the poem.
The speaker questions when all these churches will be overtaken by nature save for several which might be kept for historical significance. In the fourth stanza, the speakers wonders if there will be people who visit them for superstitious reasons such as luck, to cure illness, or to see the dead, but determines that even these superstitions will give way to intelligence and reasoning, causing churches to be overtaken by "grass" and "weedy pavement."
The speaker goes on to wonder who the very last person might be who visits a church. Will it be a Christmas fanatic, a treasure hunter, or an agnostic such as himself, who merely goes because this was the place where people got married and buried the dead.
Finally, the speaker suggests that, while he doesn't have this compulsion, there will always be people who do and will attend churches because of a need to justify their existence. Religion recognizes that need and supplies a larger meaning to life and eternal life for those who have such need.