Question is inspired by Game of Thrones, some mouseover space for a mild spoiler;
In the show, set in a medieval fantasy world, one of the characters Daenerys Targaryen is a staunch abolitionist in a violent John Brown kinda way and grants manumission to any slaves she comes across. Got me thinking - how much of an anachronism would this have been in reality? Someone who thought the institution of slavery was an evil that should be destroyed in a world full of it?
Obviously the 19th century is when abolitionism got its legs - were there significant figures for the movement long before that, even in antiquity?
The earliest I can find, hopefully you can help me find some more, is Gregory of Nyssa, a bishop who lived in the fourth century and condemned slavery in religious terms (much as the more famous abolitionists like Wilberforce and indeed Brown would centuries later).