Serfdom is slavery--you're forced to work, and you're sold with the land. That it's not exactly the same as chattel slavery does not make it any less horrible. If you are owned by people who can decide your fate, you're owned by people who can decide your fate. It was a very good thing when serfdom ended in Russia.

The Bible is fascinating in that it both condones and opposes slavery, depending on the writer. I personally go with Jesus instead of Paul: "Call no man master" leaves no room for owning slaves.

Major opposition to slavery begins with the Age of Enlightenment, I think, and includes atheists as well as religious people.

Oh, I did write about Tibetan slavery/serfdom: https://medium.com/history-and-identity/tibetan-slavery-was-not-better-than-american-slavery-it-was-differently-abominable-5df988b5fcae