>>554463
There's a series of documentaries called "Inside the Medieval Mind" you might be interested in watching if you can find it. Also Terry Jones's series "Medieval Lives"
>How powerful was Christianity in the average Western European's psyche in the late Medieval era?
Very. Have you read any books from the periods you're interested in? To them religion wasn't an add on, it was at the center of everything however it was interpreted with God read in the stars and Satan around every corner. The first books on zoology were about what animals could teach you about God for example. The fighting manuals show monks sparring, or end with how to pray for your fallen enemies. Even the artists studied nudes to further understand God because man was made in God's image. Christianity formed the entire world they lived in.
>How many of them actually followed its creed, how much sin was around compared to now?
The creed made the society so it was in the law. There doesn't seem to have been much doubt from the writings. How genuine they were who knows. Hard to say how much sin there was too and it depends on where and when. From what the paintings preach against I'd say drunkenness was an issue. Greed and vanity as well. I've seen art warning about gossip too. Very ordinary stuff.
I seriously doubt they fornicated as much as we do because of how stigmatized it was, but that's a guess. Morality was also different, knights raping the lady accompanying an enemy knight they defeated was a thing, at least in the early medieval period if I remember correctly, and in general of course there was more violence than we see today.
Another thing that's interesting is reading Dante's Divine Comedy the worst sin wasn't murder, it was treachery and betrayal. We definitely see things differently with more brutal crimes giving us a stronger emotional reaction now.
Sin had to vary from time to time and place to place. Still I can't imagine them being as degenerate as we are today. They couldn't afford to be and survive.
There was also that whole fiasco with "indulgences" which you might want to look into if you're interested in late medieval sins. Maybe it was rampant since it created an industry.
>Do you think you as a modern Christian could even comprehend that kind of immersive culture where Christianity was so thoroughly pervasive?
Maybe. It'd be a different set of rules than we're used too, possibly fewer counter to our own beliefs depending on the Christian. You wouldn't dare take the Lord's name in vain, but you wouldn't have to worry about saying something mean about fags or muslims getting you fired or listen to blaspheming fedoras anymore. Some might find it refreshing.
>what context/scenario would be appropriate for a painted rendition of one on a military tool like a shield?
There isn't one. They didn't paint demons on shields they painted standards. Images that signified who you were and what family you were a part of so when people saw you do something heroic it added to your reputation, and if you ran away you were shamed. A demon would be like painting "I and my family love Satan, please burn us to death" on the shield. The closest you'll find is probably griffons and dragons.
The concept of a "black knight" I've heard comes from knights who'd black their shields out because they were about to do something nasty and didn't want anyone to know who they were. Maybe one who felt bad about what he was doing might brush a crude demon on it. Just an idea.