The great mystery of the birth of Christ is probably most famously depicted in Tomas Luis de Victoria’s setting of O magnum mysterum for Cantus, Altus, Tenor, and Bassus using his somewhat mystical, Spanish Renaissance tune.  These variations are adapted from his work and would do nicely for the prelude on Christmas Eve, for Lessons and Carols or for any Sunday during Christmas.  Most organists will find it moderately easy and a nice addition to their Christmas repertoire.