Nobody can look upon virtue and justice, if it be only in images and pictures, without feeling a kindred sentiment. Nobody can be surrounded by vice and wrong, by violence and brutality, if it be only in images and pictures, without coming under their degrading influence. Nobody can live with the one without advantage; nobody can live with the other without loss. Who could pass his life in the secret chamber where are gathered the impure relics of Pompeii, without becoming indifferent to loathsome things? But if these loathsome thing are not merely sculptured and painted if they exist in living reality - if they enact their hideous capers in life, as in the criminal pretensions of Slavery - while the lash plays and the blood spirts - while women are whipped and children are sold - while marriage is polluted and annulled - while the parental tie is rudely torn - while honest gains are filched or robbed - while the soul itself is shut down in all the darkness of ignorance, and while God himself is defied in the pretension that man can have property in his fellow man; if all these thing are present, not merely in images and pictures but in reality, their influence on character must be incalculable. ...The honorable slaveowners were not much pleased with Sen. Sumner's descriptions of them. Even though those descriptions were accurate.
Instead of "ennobling" the master, nothing can be clearer than that the slave drags his master down; and this process begins in childhood and is continued through life. Living much in association with his slave, the master finds nothing to remind him of his own deficiencies, to prompt his ambition or excite his shame. Without these provocations to virtue, and without an elevated example, he naturally shares the barbarism of the society which he keeps. Thus, the very inferiority which the slave-master attributes to the African race explains the melancholy condition of the communities in which his degradation is declared by law.
A single false principle or vicious thought may degrade a character otherwise blameless; and this is practically true of the slave master. Accustomed to regard men as property, his sensibilities are blunted and his moral sense is obscured. He consents to acts from which civilization recoils. The early church sold its property, and even its sacred vessels, for the redemption of captives. This was done on a remarkable occasion by St. Ambrose, and successive canons confirmed the example. But in the Slave States this is all reversed. Slaves there are often sold as the property of the Church, and an instance is related of a slave sold in South Carolina in order to buy plate for the communion table. Who can calculate the effect of such an example?
The New York Times has the text of the speech online. I rely here on the text from the version published in 1863 as Barbarism of Slavery.