I can agree with some of the sentiments of this article, but there are some rather preposterous errors in it.
"Slavery legitimated through Christianity"
That seems to be a rather absurd idea. Slavery was pretty much universal in the ancient world. If anything, Christianity did a lot for slaves in antiquity by insisting on fair treatment. Sadly it failed to abolish it, but that had more to do with them having a hard time imagining how society would work without it (Gregory of Nyssa for example was in principle opposed to slavery). Abolishing slavery was ultimately rooted in Christian thought. Sadly it took way too long.
Natural law is also not an originally Christian concept, it is Aristotelian.
Here's the problem, when you claim your religion came from god, you can't then explain away the fact that it allows for slavery by saying it was everywhere at the time or because people didn't know how to make society work without it. Your laws were made by god, remember? Are you telling me god didn't know any better or that god couldn't find another way to make society work? Once you claim your religion is inerrant, it's not good enough to say it was better than average relative to its day and age.