I didn't say commend, I said condone. The Bible regulated slavery, which means it allowed slavery to exist and orders that slaves to be obedient. It never condemned slavery as modern society does.
But "modern" society, as you put it, condemns slavery only because all of us --secular people as well --- are living off of the moral capital of the Judeo-Christian "deposit of faith." To the extent that that deposit dwindles over time, not being renewed by faith, the moral structure becomes vitiated and finally disappears.
As Judeo-Christianity recedes, you've got Islam advancing with its frank endorsement of Koranic slavery which has never been repudiated in the Muslim world; you've got totalitarian slavery in North Korea and allied dictaorships; you've got human trafficking and sex-slavery in the West; and you've got the redefinition of human life as a product and not a person (e.g. through genetic engineering) which provides the technological preconditions for the New Slavery of the Future in which everyone will be a product and not a person.
Without reference at least to the Creationist-Deism of Jefferson ("all men are Created equal.. are endowed by their Creator with certain inalienble rights...") we have no axioms to rely upon which could supply a basis for a theory of human rights.
Slavery has been a given in almost all human societies. In the whole history of global cultures, it's the lack of slavery in our own society is an anomaly.
Why are Christian (and, for a while, post-Christian) societies so exceptional? For the answer, read a bit of Rodney Stark's book For the Glory of God: How Monotheism Led to Reformations, Science, Witch-Hunts, and the End of Slavery or his other fascinating book, The Victory of Reason: How Christianity Led to Freedom, Capitalism, and Western Success.