While it might not be as big of a deal in "Church Going" as religion or spirituality, the relationship between humanity and nature helps Larkin explore the conflict between the sense of order that humanity tries to force onto the natural world, and the indifference that nature has to this sort of effort. At several points in this poem, the natural world serves as a foil to religion, since nature is a symbol of the inevitable decay that happens to everything that humans try to impose, whether it's something physical like a church, or non-physical like Christian beliefs. It may not seem like it—heck, we may not even want to admit it—but humanity is a fleeting thing, and so are its attempts to mold the world into its own image. Nature, on the other hand, will keep on living long after we're dead. For this reason, the images of nature in this poem often have almost a post-apocalyptic feel to them, reflecting a world in which humanity and human forms of meaning are totally gone.
In "Church Going," Larkin demonstrates that all of humanity's attempts to make the world feel like a meaningful place will ultimately fail in the face of nature's cold indifference. So take that, puny humans.
Pack your tents, folks. In "Church Going," Larkin shows us that whether we have faith or not, we'll never feel spiritually fulfilled until we start to be one with nature.