I have a friend who is a prof in a state school.
She began to notice the lack of religious and cultural knowledge around 15 years ago. She said that she could at least count on the Catholics until about 7 years ago.
Now when she teachers her arts and philosopy classes she has to make sure that she teaches the basic tenents of christianity so the students can understand western art and western philosophy.
The ignorance you describe isn’t by accident but by design. And that is frightening.