TLDR: The actual number of self-reported religious folks used to be higher than the actual number of religious folks.
I'd tell a story about me being in church, and people would say something like, "Oh, I'm a Christian too."
I'd ask what church they go to, and I'd get the deer-in-the-headlights look. It wasn't meant as a challenge; it felt like the best follow-up question to their response.
Nowadays, more people are more likely to say they aren't religious.
Since a lot of the research is simply polling people on their religious views, the number has been inflated for a long time.
If we actually looked at religious attendance, like counted the number of people in a town and counted the number of people walking into a religious building in a week we'd have a far more accurate number.