When did child sexual abuse start to be considered one of the most deplorable crimes? If you asked someone 200 years ago what they believed to be the worst crime to commit, what would have been their response?

NSFW, trigger warning, &c

Explicit concern over the "psychological" impact of childhood sexual abuse is a modern phenomenon in the West, 20th century even. But condemnation of rape and sexual assault of children is ancient, and not just Christian. In this earlier answer and follow-ups, I discuss how late antique authors from a variety of religious backgrounds recognize spiritual and physical harm resulting from the sexual exploitation of young people, even when approved by other elements of society.

You might look at that answer and say, "Well, that's really not a lot of evidence; John Martens had to do a lot of work to show that the Bible--the Bible, for goodness sake--condemns child abuse. So sure, they saw it as a problem, but it can't have been that big a deal. I mean, Pseudo-Paul writes more against braided hair."

And it's undeniable--anyone trying to learn about sexual crimes/perception of sexual crimes in Western history almost immediately hits a massive roadblock: there is an enduring taboo against talking about them. However, the way they are not talked about--which is to say, the scraps that they are--gives us keen insight into the emotionally devastating power the idea of sexual crimes held over people's imagination. I'm going to focus primarily on evidence from medieval western Europe here because that's my area of expertise.

Already in the Hebrew Bible, which is significant for its originating culture but also in influence, there are three explicit rape narratives (I'll leave aside the scholars' debate over Sodom and Gomorrah). In none of them is the focus on the rape or the woman. Instead, the stories use the rape to explain--to justify?--the subsequent violence of men. The stories portray the terrible shock of rape to the community. On one hand, it's a pretty damn misogynist view to say "rape is really a crime against the men connected to a woman"; on the other, the stories illustrate that with rape as a proxy crime against men, it can matter in a way crimes against women cannot matter. (Hold both your objections; I'll get there).

Medieval sources from the Latin Christian and Islamic worlds have to confront rape in parallel contexts: war. In this answer, I show that Latin communities dealt with plunder-by-rape by basically not dealing with it. The practice was so ubiquitous that communities who made it through a Viking (in this case, but it is generalizable) attack with members still alive just kind of assumed married women who got pregnant and did not lose the pregnancy were giving birth to legitimate children. That's not to say rape "didn't matter." 14-15th century humanist Christine de Pisan calls being raped and living with it afterwards "the worst possible sadness." This woman--who lost her husband and knew a little something about grief--compares the experience to mourning. Mourning what was not just lost but stolen. Christine is the only medieval woman to write about this topic directly. In the Middle Ages, you don't talk about rape.

Except when you do. And when you do is when you are going to paint your enemies as the worst possible barbarians. We don't see a lot of this in early medieval ecclesiastical chroniclers raging about Vikings, because the major group of women victims they discuss are nuns. And nuns' chastity HAD to be protected at ALL costs, even rhetorically. (Jerome, who translated the Vulgate, argued that it was better for virgins to commit suicide rather than "let" themselves be raped. Although Augustine argued contra and technically his view was accepted, traces of Jerome thrive in the medieval West.) But we get to see inside the horror that rape presented to the Christian and Muslim mind alike in writing about the Crusades:

In Robert of Rheims' account of the initial call to crusade at Clermont, he writes, "What can I say about the evil rape of women [by Muslims], of which it is worse to speak than to be silent?"
Guibert of Nogent obliquely references how the Saracens spare attractive young women to be slaves
Albert of Aachen goes further and describes the women prisoners of war as, explicitly, "virgins" (so, young women and girls)--and also boys, by the way: "They took away only young virgins and nuns, whose faces and figures seemed pleasing to their eyes, and beardless and attractive [male] youths."
Muslim chronicler Usamah ibn-Minqidh writes that when his hometown of Shaizar was under attack by crusaders, he and his mother would go out onto their home's balcony "so that in case [they] reached us, I could push her and throw her into the valley, preferring to see her dead rather than to see her captive in the hands of the peasants and rapists."

Rape is seen as so bad that it serves as a rhetorical weapon, in addition to being an actual one. Everyone knows that their side does it, too. But they can't talk about it. However, it's the rhetorically-perfect escalation of the enemy's barbarity.

I want to draw attention to Albert of Aachen's Historia, where he writes that 'Saracen' victors kidnapped girls and young women but also teenage boys into sexual slavery. Medievalists might be pretty floored at this quote, because they did not talk about men as rape victims in the Middle Ages. They absolutely just did not.

Even sodomy, as William Peraldus tagged it in the 12th century, was the "sin that cannot be named"--and that's consensual. The spiritual and social ramifications of rape for a male survivor were too unbearable to put into words.

Ancient, late antique, medieval, early modern Europe and the Near East had long, long lists of horrible crimes. Heresy, infanticide, forgery, treason (Dante Alighieri's Inferno has some pointers on ranking sins/crimes, for sure), murder of one's family. As with rape, you can find cases where these crimes were punished lightly or not, where empathy and mercy were shown to the perpetrators or not. However, in terms of emotional impact and heft as a rhetorical and martial weapon, sexual crimes have a long history of pitch-black supremacy in western traditions. I'm not sure it is merely irony that the virulent condemnation of rape in rhetoric has had less of an impact on treatment of sexual predators versus their victims than we might think. Rather, as the medieval chroniclers' treatment of wartime rape by their co-religionists versus religious enemies suggests: By making sexual exploitation the crime of ultimate depravity and ultimate barbarity that only Other people could possibly commit, we can more easily--safely--comfortably--fail to confront it in all its degrees among our friends, families, allies, and the people in power we think might help us out.