(Just to explain, the original title was a tad confusing, which is why the exchange with @aubeynarf@lemmynsfw.com might seem a bit weird.
Sorry for that. I modified the title to clarify it, and so hopefully it does make more sense, even if it now makes the post from aubeynarf look a bit odd (for which I apologise).
From a purely secular-civil standpoint, an important function of religion is to offer solace and comfort regarding the finite nature of life.
Folk who repeat the phrase with sincerity are professing a belief in the God alluded to in the phrase, who watches over each human specifically. For those believers any obstacle which occurs is either a challenge they are meant to overcome or a fate they should accept the way Jesus of Nazareth accepted crucifixion.
For those who don’t believe in Jesus, the God of Abraham, or that They are micromanaging our lives, the phrase is an annoying thought terminating cliche and dismissive platitude.
I’m not sure slapping them is necessarily appropriate, though.
It would be acceptable under their belief system because it was God’s will that they were slapped and the pain is an obstacle they must overcome.
If they’re truly devout, they’ll turn the other cheek.