In fact, in the Protestant Evangelical Christian world I was raised in, back in the 1970s, secular was already a synonym for evil. Calling somebody secular was pretty much the same thing as calling them demonic.

As a child, I heard things like:

"No, Johnny can't come over to play again. His family are too secular, and we can't have that influencing you."

"No, we don't want you going to public school with all those secular children."

Etc