The answer to this question is rather complicated. But, as a general context for "Western Slavery," we must begin where it began; Ancient Rome. Slavery was an integral part of Roman and Hellenic society during the ancient and classical eras. It was said that "philosphers" were integral to the conceptualization of "Western Slavery," and, it was no secret that Aristotle and Plato were strong advocates of slavery as a "natural" human societal dynamic. Catapult yourself into the 16th, 17th, 18th, and 19th centuries, and what you witness in the Americas is fundamentally, a systemic belief system that was practiced by the "dominant" here as the inheritors of Romantic and Hellenic cultural/philosophical ideology. And it is poignant to remark here, that even Thomas Jefferson had or owned slaves.
Now, in Christianity Peter, several times in the Bible itself, never denounced the practice of slavery, in fact Peter (the principal architect for ecclesiastical "law") spoke of actually "regulating" it! You must understand that the statement that "all people are children of God" is not a statement on what those "children" are. Similarly, when we look at the Articles of Confederation and the Constitution, when can plainly realize that slavery existed when these documents were created, and that the meaning of the words in them did not include a specific defining of who actually were "all men" that were "created equal."
What you are really looking for is not so much a clarification of concepts that speak to a disconnected behavior to beleif, but, who actually "makes" the belief to begin with. This is very important as a distinction, for you may find, that the only consistent "thing" that circumscribes Christianity AND the existence of slavery, are dominant "males" who govern the "Western" world of ideas and thoughts. Its just that simple. If you pay too much attention to the words as if they represent what is actually occurring, you will miss what is ACTUALLY occurring.
What is "legal" is only that which defines a "majority" of a given societie's standard for behavior. In this sense anything can be legal, hence the existence of slavery. The Constitution even protected former slave owners and the slave system, by denying the right for people to consider that behavior criminal if a new law came into existence that did not exist when a crime was commited. That new law was that slavery became illegal.
You must understand that all things in the "Western World" do not just spontaneously appear, but come from those who are in the "majority" positions to dominate. Again If you pay to must attention to words and language that seems to represent what is actual, you will miss what is ACTUALLY occurring and what that means as its own "revelation." Sadly, that is the state of most people in "Western Culture." Well, I hope this helps; take care!