How was slavery in the U.S. justified by Christianity? President Barack Obama recently stated that many atrocities were done in the name of Christ.  He went on to say that Slavery in the United States was also justified by Christianity.

Since he did not cite any Scripture, in his statement; I have searched the Gospels for any words of Jesus, which could be construed to support slavery; and have found none.

If Slavery was so justified as being defended in the name of Christ, I cannot find it.

Can anyone assist me in understanding the President's assertion?
Robert Bruce Mullin says in 'North America', published in A World History of Christianity (edited by Adrian Hastings), pages 437-8, that Southern religious figures claimed not only that was slavery a positive good, but that it was a Christian institution. 

This slavery argument gained a specific religious dimension, and the northern Protestants faced a difficult question - how was slavery a sin? Did the Bible ever condemn slavery as sinful in its very nature, and if not, were there actions which were in fact sinful which the Bible did not recognise as sin? A close reading of Scripture demonstrated that the biblical authors seemed to accept slavery as an institution and nowhere explicitly condemned it, and Mullin says this theological question became a troubling aspect of the American crisis. He cites the Presbyterian theologian Charles Hodge (1797-1878), who insisted, “nothing is obligatory upon the conscience but what [the Bible] enjoins; nothing can be sin but what it condemns.”