The writers of the New Testament lived in a different world. They weren't much above slaves themselves in the Roman world, so they had to watch how they reacted with their surrounding culture as they established the new faith.
The early Christians were free people. A strong distinction was made in the Roman era between slave and free, just as it was centuries before, and centuries later. Yes, the general population of Judea may not have had great status, but then neither did the general population of any province of Rome. However, it so happens that these people had been practicing slavery themselves for a long time before they became part of the Roman empire, and understood quite well the distinction between slave and free, and
knew themselves to be free. The slave-holding aspect of the "surrounding culture" was not foreign to these people; their holy book condoned slavery before Paul ever put pen to paper.
It seems that they adopted the principle of "don't make waves, unless it's about an ultimate issue." They were prepared to die for their faith, but didn't try to change every evil practice in the Roman empire.
The practice of slavery, as I pointed out above, was not something that the Romans had imported to the area. It seems you're trying to imply that the poor proto-Christians had to meekly accept the ways of the evil Romans. Their ancestors had been proud slave-makers and slave-owners long before the arrival of the Romans, so that particular assertion falls by the wayside.
It all depends on how you see the Bible - if it's the inerrant Word of God, then you have to contend with the charge that it promotes and cndones some bad stuff. If it's just a written record of people who have experienced God in some way and they are trying to relate that in terms of their own culture, then stuff like slavery is not as big an issue. It's just how those people worked their faith out in their world. We might do it 100% differently today.
Just people telling us stories; no divine revelation? We can consider a couple of things then.
1. The distinct possibility that none of the gospels was in truth written by an eye witness.
2. The epistles tell us that Paul never actually met Jesus except in his self professed vision of a presence which he said was Jesus.
In light of these circumstances, why give the New Testament any weight at all as a source of moral guidance? The Old Testament is at least as dubious if we dispense with a supposed direct revelation from YHVH, I think. Does Christianity have any moral authority, according to this view? If it does, from whence does it derive? I don't think that the say-so of some 1st century true believers quite does it. The say-so of their predecessors really isn't sufficient, either.
We can also consider something that Thomas Paine pointed out: "It is from the Bible that man has learned cruelty, rapine, and murder; for
the belief of a cruel God makes a cruel man." (
Letter dated May 12, 1797 my emphasis) I don't agree that man has learned cruelty from the Bible, but it could reasonably be said that man was and is able to justify cruelty by recourse to the Bible.