This is Laetare Jerusalem ("Rejoice Jerusalem") sung by Discantus in early polyphony. Last Sunday was Laetare Sunday--a time when we lighten the somber mood of Lent a bit and rejoice that our salvation is near at hand.
"Slavery ended in medieval Europe only because the church extended its sacraments to all slaves and then managed to impose a ban on the enslavement of Christians (and of Jews). Within the context of medieval Europe, that prohibition was effectively a rule of universal abolition. "— Rodney Stark