Peebo, you need to start another thread for that. The Spanish Inquisition happened in the “Early Modern” period, not in any kind of “Dark Ages.”
Every parish church had a priest’s missal and a ectionary book, with all the readings and prayers. Every priest read the prayers from the missal and the readings from the lectionary. Sometimes they weren’t terribly good at it, and there were a few scandalous cases where priests apparently just mumbled, but the fact they were a scandal tells you how few they were.
Bishops either trained and educated priest candidates at their own house (the beginning of seminaries) or at a cathedral school. Later on, priest candidates went to universities. All priests had to be personally ordained by some bishop, or by an abbot with the powers of a bishop. They didn’t just pick people out at random.
In a small village, the priest might be the only one who knew how to read and figure (ie, do math), so he was often called upon by the whole village and its lord, to write letters and do accounts. This was a pain in the butt, so priests had a lot of incentive to teach these basic skills to both adults and children.
In anything bigger than a village, the lord’s steward would know how to read and write. So would any merchants or prosperous farmers. So would lawyers and physicians. Often their wives and kids would also know how to read and write.
Nuns often knew how to read and write, because that was an important part of a convent education. They had to know Latin so as to sing the Hours and the Mass. Nuns also copied out and illuminated books, just as monks did. Recent research has revealed that they were doing a lot of the big Carolingian copying project, for example, but even in the Dark Ages, they show up as copyists and illuminators.
The Franks actually tended to educate normal laywomen living at home more highly than the men, because men were expendable but women tended to live. It was the job of women to run the household and cure the sick, and an educated woman brought a much higher brideprice to the clan. (And she’d be coming back to her birth family in the end, if she lived, so no investment in her education would be lost to her family by marriage, unless she died in childbirth or the like.)
Education was time-consuming and expensive, but it also tended to spread. There was less schooling in times of turmoil, and more in times of peace. Knowledge never died out entirely.
But yes, the average Dark Ages peasant probably knew more Bible stories and sayings and proverbs than the average American who claims to be Christian. They had retentive memories and not a lot of entertainment to distract them. So they actually paid attention in church.