O Come, Let Us Sing Unto The Lord was written for Stephen Layton and Polyphony by Morten Lauridsen.  Purchase the sheet music for unaccompanied SATB chorus. 
In speaking of Lauridsen in his book Choral Music in the Twentieth Century, the author Nick Strimple says of him, 'he is the only American composer in history who can be called mystic, (whose) probing, serene work contains all the elusive and indefinable ingredient which leaves the impression that all questions have been answered... From 1993 Lauridsen's music rapidly increased in international popularity, and by the centuy's end he had eclipsed Tandall Thompson as the most frequently performed American choral composer'.