I posted about this some time ago, and thought it might be apt to recall its significance both in the light of today's Feast, and also of recent events, particularly the atrocity in Woolwich a few days ago.
The White Scapular is the symbol of the Trinitarian order, and arose from the vision of St. John de Matha, in which he saw Christ chained between a Moor and a Christian. The order he founded, along with St. Felix de Valois, was originally aimed at the liberation of Christians who were at the time being held captive by the Moors in North Africa. Perhaps today the White Scapular could serve as a devotion against the sort of extremism which leads to events such as the murder in Woolwich.
Those enrolled with the scapular benefit from the prayers and privileges of the Trinitarian order, and can gain associated indulgences. The White Scapular forms part of the Five-Fold Scapular, and can also be substituted by the Scapular Medal. For more information about scapulars, see this article.