I heard it for the first time a couple of days ago on mn. As an actual real Christian I'm not that keen on it in a very mild, turn the other cheek way, you understand
That really made me laugh.
But you see, I don't think that it's a new thing (maybe a newish term) or a choice or a movement.
It's more of a fact isn't it? A background thing?
You (or I or anyone) could abandon every single scrap of belief or actual religious practise we ever had, but the scripture, hymns, habits would always stay with you. You can't escape it.
Those things are obviously more meaningful to some non-believers than others, so those people are more likely to call themselves "culturally christian" maybe? The influence is there, whether you value it or not, though.
The same has happened on a societal level.