Slaves in ancient Israel were more like servants than slaves as we think of them in the American context. Also, that's part of the law that only applied to ancient Israel. Christian countries were at the forefront of abolitionist efforts.
Leviticus states that slaves may be purchased, treated as property, their offspring treated as property and passed to your children as a 'permanent inheritance'
That is slavery.
A female slave that does not 'please' the man that bought her can be bought back again.
That is sex slavery.
It is slavery; arguing over the nuances or saying it wasn't 'as bad' doesn't detract from what it was.
On Christian countries being at the forefront of abolitionist efforts, religious and non religious people and groups within them were yes. But Christian countries (citing religious justification) were also at the forefront of retaining slavery for as long as was useful for them.