O Magnum Mysterium by Tomas Luis de Victoria is a Catholic chant sung at Christmastime. As part of the Matins, the text has been around for centuries. While the exact origins are unknown, historians believe the text has been around since at least the 10th century.
Victoria is one of the most well-known composers of the Counter-Reformation He was born and grew up in the Spanish city of Avila became known for his musicianship at a young age. O Magnum Mysterium was first published in 1572 as a choral motet.
In 2018 Reading Phoenix Choir visited the city of Avila as part of the Spanish Tour. As well as singing the O Magnum Mysterium in the Auditorio de San Francisco, choir enjoyed lunch at a restaurant which was allegedly the house where Victoria once lived!