"Church Going" draws a pretty clear line between spirituality and religion. Spirituality is the part of the speaker that keeps drawing him back into churches, even though he doesn't find anything in organized religion. In this sense, spirituality refers to the basic human longing that leads people toward religion. The poem describes this longing as a profound desire to be serious and to have a serious meaning in your life. Otherwise, life is just a big joke. Not the funny kind either. More like an old, dusty, knock-knock kind. For this reason, the speaker implies, spirituality will always exist even if religion doesn't. Religion means knowing all of the customs and rules of a specific faith, while spirituality, as the speaker shows us, can be vague and "uninformed" (46). According to Larkin, religion provides hard answers to life's big questions, while spirituality is what keeps us asking these questions. In this sense, you might say that Larkin doesn't necessarily endorse religion, but he definitely finds something in the idea of spirituality.
While "Church Going" doesn't give a very flattering portrait of religion, it downright celebrates spirituality. Woo-hoo!
Ultimately, Philip Larkin's "Church Going" suggests that just like religion, spirituality will eventually crumble and disappear, leaving nothing behind. Bummer.