The "cultural Christianity" argument says that atheists might not like Christianity, but they like a culture which depends on Christianity. They like open, free, thoughtful, liberal, beautiful, virtuous societies. Unmoored from a connection to Christanity, a society will gradually have less of those goods, until even atheists are unhappy.

Therefore (continues the argument), atheists should be cultural Christians. While they can continue to privately disbelieve, they should support an overall Christian society, which they can dwell contentedly on the fringes of. I think this is sort of where Ayaan Hirsi Ali is coming from.

https://readscottalexander.com/posts/acx-against-the-cultural-christianity