Not sure what you mean by 'Christian religions' (multiple?) or 'biblical times' as that covers a potential 2000 year block of history but assuming you just want documentation, then the recent Cambridge Slavery in the Ancient Near East has two lists of original documents, one of ancient documents (more Old Testament, and one of New Testament times which is pretty much exhaustive. Those links might be down for maintenance, in which case try the general link for the book.

Chirichigno's Debt Slavery in Israel and the Ancient Near East is a good start for the OT, and Isaac Mendelsohn's Slavery in the Ancient Near East (they're not very imaginative with titles) would be a good, if old companion to general ANE slavery. If you have Jstor access, then you could try Baker.

Otherwise Thomas Wiedemann's Greek and Roman Slavery: A Sourcebook has a good selection of primary sources for NT times.

There is, unfortunately, nothing comparable to nineteenth century slave narratives. We do have the works of freedmen (such as Horace) and the vigorous epigraphic habit of freedmen as a class means we know a great deal about them and their ideology, and it seems extremely different from New World slaves. However, this has the built in selection bias of only giving us the perspective of the succesful slaves and ex-slaves.

In general, the most important principle of ancient slavery is its sheer diversity. It is true that slaves in the American south could and did fulfill a variety of roles, but in the ancient world a slave could be anything from the most menial, expendable laborer to a leading official in the Imperial administration. There were trading colonies established and largely inhabited by slaves, for example, and a large portion of the revolution in Late Republican intellectual culture can be attributed to the importation of highly educated Greek slaves, who produced works and taught elite Romans. There was also real and attainable hopes for freedom an success afterwards, more legal protection and a degree of personal autonomy.

That being said, I dislike saying ancient slavery was "better" because that puts the shoe on the wrong foot. New World slavery was worse, it was an unusual institution, and that should be remembered.

The Roman Equestrian, Vedius Pollio, fed his slaves to lamprey eels while they were still alive. While it was considered excessive, it shows the extent of cruelty that could occur.

Not to mention the mines...