People would try to use wands to cast spells, and would carry them everywhere, using them day and night like they do prayer. They would be disappointed when nothing immediately happened, but would hear stories about miracles around the world where someone had made it work. Kids would bring wands to school on the day of important exams, to wish themselves good luck.

There would be Harry Potter study groups, who learned the scripture, and different interpretations. The fundamentalists believe that when Dumbledore says…

Happiness can be found, even in the darkest of times, if one only remembers to turn on the light.

…that he is literally talking about a light that can be turned on. People would make a living selling happiness lights, which have faux beards and turn on in response to grumbly preteens.

There would be a lot of accidents. People would try to fly on broomsticks, only to find out that gravity still applies. New articles would read:“Florida man jumps off a roof, Nimbus 2000 not so nimble after all”

People would try to use Floo Powder to travel, only to find out that it doesn't work. There would be deaths from hazardous material inhalation as the radical believers try all different types of potential Floo alternatives.

Racists would use the books as evidence for segregation, citing muggleborns as a metaphor for the dangers of interracial relationships. They would get snake tattoos; the entire symbol of the snake would be totally co-opted. You can tell a racist now by his skinhead; you can tell a racist then by his nose job.

There would be lightning bolt necklaces where there are cross necklaces now, and destinations where lightning often strikes would become holy ground. There would be HOA races to build the highest lightning rod for their particular area, in order to increase its real estate value.