Composers like Aaron Copland, William Schuman, Roy Harris, and Ned Rorem not all of them sympathetic to Barber’s music in general look at this work and shake their heads, wondering how he pulled it off. They fall back on phrases like "finely felt," "poetic," "nothing phoney," "a love affair." There’s no real complication to the Adagio, no technique or unusual turn of harmony that holds the secret of its success. One cannot even pick one passage over another, any more than you can say one point makes the beauty of an arch. This is a masterpiece.
Source: Classical Net – Barber – Adagio
Samuel Barber’s "Agnus Dei" is his setting of his own Adagio for strings. This performance of it is by the English choir Ex Cathedra conducted by Jeffrey Skidmore. It’s the most beautiful recording of it I know of.