I disagree. While there would be differences, of course, I believe the difference between, say, modern America and South African bushmen is far, far greater. After all, we're talking about a human society, here, not something on Zeta Reticulae, and humans are humans. Rome was a different society, yes, but the temporal distance is no more "distorting" than would be a geographic distance and, after all, Roman society was the ancestor of our own. The differences are magnified by the similarties.
So what?
I must first caution you that you've struck a nerve. I, myself, am a non-believer, and I quarrel with the idea that you must be a believer of any sort in order to have an ethical, moral or social referent.
As to Christianity "ameliorating" Roman society, I believe it is not too far-fetched to argue that Christianity was responsible for destroying that part of the Roman ethos that made possible the very construction and maintenance of society. In short, I hold that Christianity destroyed the Roman spirit and turned the people into something the Founders would have scorned, and maybe even something the Founders would have conquered.
Getting back to Christianity, the early Church had no problem with slavery. I believe it's in Philemon where Paul urges a slave to return to his master and render faithful service. If slavery dehumanizes, the early Christians were just as dehumanized as were the Romans (given later developments, maybe they were even more dehumanized).
No more so than in 19th century western United States, or in any other society that has not had access to late 20th century medicine.
...has been the norm throughout history and, when numbers are taken into account worldwide, may still be the norm.
Shocked and scandalized, yes, but no more than the Victorians would be shocked and scandalized by us. And understanding would occur to anyone who actually sat and studied the differences.