This famous poem is so rich in terms of the title that Larkin has given it and the number of possible interpretations and how those interpretations relate to the theme and message of the poem. Ostensibly, the "Church Going" refers to precisely the kind of activity that the speaker of the poem is engaging in. As churches have fallen in prominence thanks to the secularisation of society, England is (still) littered with thousands of small country churches that aren't really used for their original intentions any more. Church going therefore becomes a kind of hobby that the speaker awkwardly indulges in.
However, church going also refers to the gradual slide towards secularisation in society that has resulted in the gradual fossilisation of religion and church as a society. The speaker often finds himself, after his compulsive church visiting, questioning the purpose and role of church in society:
When churches will fall completely out of use,
What we shall turn them into, if we shall keep
A few cathedrals chronically on show...
Meditatively, the speaker begins to ponder the felt needs that places such as churches meet and the way that the secularisation of society still leaves even avowed atheists himself with some kind of hankering after the eternal:
A serious house on serious earth it is,
In whose blent air all our compulsions meet,
Are recognised, and robed as destinies.
There is something of the human condition that hungers for the divine, and churches, even though they are "going" in terms of sliding away, become a perfect place to meet those needs and ponder those eternal questions. Ironically, therefore, the title both points towards the importance of churches and how vital they are to humans as well as their demise.