O magnum mysterium

The second part of the responsory is a versicle which is based on the words with which Elizabeth welcomes Mary on her visitation (Lk 1.42–43).

David Thompson states that "the origin of this Nativity poem is in itself a great mystery.

The poem/chant was incorporated in mediaeval times into the Divine Office as the fourth of the nine Responsories for Matins on Christmas Day."

He conjecturally dates its incorporation to the liturgical reforms of Pope Gregory VII.

Some of the earliest settings are by Paolo Aretino (1508-1584), Adrian Willaert (c. 1490 – 7 December 1562) and Nicolas Gombert (c. 1495 – c. 1560), which were published in the mid 1500s.