Given the quote, it would basically be saying that a lot of these people are “cultural Christians.” They don’t adhere much to the doctrines of their church, but call themselves Christians because that’s what you’re supposed to be.
You can’t tell for sure from the outside, but there are signs. The Bible itself has some “fruits of the Spirit” that give the characteristics of someone who follows Christian doctrine. The main one I see is hatefulness.
Other things are just whether they commit obvious sins without any sign of feeling bad about it. That guy who says he’s a Christian but cheats on his wife? Not a true Christian.
I agree that the way we let people get away with it obvious sins in our society shows we’re not all that Christian. And I’m not counting freedom of religion stuff. I mean we have a society entirely run by greed.