Slavery vastly predates Christianity...or is your idea that there were no slaves in ancient Greece or Rome or Persia or Mesapotamia?
Actually, we find that slavery is as old and august an institution as prostitution and killing neighbours. It's as old as history itself, and clearly predates even that, since there would have to be slaves already if history were to record any.
That the Christian Bible uses slavery as a synonym for "bondage to sin," or that it comments on the existing practices and lives of slaves (For example, telling people, "If you are able to buy your freedom do so, and if not, you can still serve God.") does not convert slavery into a "Christian" institution, anymore than the Emancipation Proclamation makes slavery an "American" concept.