This thesis explores the relationship of the Sermo Angelicus of St. Birgitta of Sweden, written in the fourteenth century, with the Nativity/Concert of Angels panel of the Isenheim Altarpiece, painted by Matthias Grunewald in 1514 for a hospital and monastery run by the Antonite Order. Taking into consideration the context of the altarpiece, this thesis analyzes its iconography in relation to specific passages from the Sermo Angelicus, suggesting that the text was a possible source used by the Antonites in the Nativity/Concert of Angels panel. By doing so, parallel themes of salvation in both the text and the panel are discovered that in turn relate to the altarpiece in its entirety and present a message fashioned specifically for those patients at the hospital at Isenheim that viewed the altarpiece.