Don't you think "do unto others" is a principle that would apply to abolition?
But it took 1800 years for it to happen. And there is zero in the Bible condemning slavery. There is do under others in the Bible but I guess the Christians said except slaves for 1800 years.
One the the first writers in the colonies to condemn slavery was Thomas Paine and was a founding member of the first anti slavery organization. Paine was for sure not a Christian.
https://www.constitution.org/tp/afri.htm