It means that people who claim they adhere to x religion but they just pick and chose the parts they like (which is the norm), aren't really taking to heart the teachings of x religion. It's also a somewhat recent phenomenon (although that too depends on what society we are talking about). These people mostly just use religion to justify their personal opinions and the social mores they learned to obey to themselves. But these things are totally irrelevant when you supposedly believe there is a supernatural being in charge of the world who wants you to have a certain purpose.

This is evidenced by the fact that most of these people usually have never studied the holy texts of the religions they adhere to, and in general spend little time figuring out what their religion is even supposed to say. I don't really understand that, like, if I thought there was a God I'd be super invested in learning what the fuck he wants from me. I guess this doesn't automatically mean they don't take it seriously, but it does show that the way they learned to interact with religion is closer to something which just justifies the stuff a specific social circle thinks you should follow rather than anything religion is ostensibly about.