I think the spirit realm has long been society's means of explaining the inexplicable. In bible times, and in medieval times, "demons" were credited with causing things like retardation, seizures, multiple births (twins), etc. As we've grown up as a race, we have come to realize that these things are not in fact demons - they are natural occurrences.

I would submit that attributing those things to a spirit realm stunted our growth as society - superstition in place of learning via curiosity.

Most things credited to the spirit realm today can probably be explained by natural phenomena. In some cases it is things as simple as our body healing itself after we said a prayer for god's help in the matter. Simple coincidences. Maybe I prayed for God to reveal to me whether he died on a cross or a stake - the next few day, because my senses are tuned in to the matter, and I am looking for evidence, I take note of many random cross-shaped things that I assume were placed there by god or his angels - interestingly, my prejudices and preconceptions prevented me from seeing all the things that looked like stakes or poles.

There are plenty of things we cannot explain about our world. But throwing up our hands and saying "Goddidit" or demons were involved stunts our intellectual growth, IMO. There is nothing supernatural out there - our natural world is amazing enough. And we are definitely still like infants trying to understand it.

Live a good life. If there are gods and they are just, then they will not care how devout you have been, but will welcome you based on the virtues you have lived by. If there are gods, but unjust, then you should not want to worship them. If there are no gods, then you will be gone, but will have lived a noble life that will live on in the memories of your loved ones. I am not afraid.
Marcus Aurelius