The text comes from the Church’s sacred liturgy: it is the Benedictus antiphon at the office of Lauds for the Feast of the Circumcision.
Learn about the composer here.
Mirabile mysterium declaratur hodie,
innovantur naturae; Deus homo factus est;
id quod fuit, permansit,
et quod non erat, assumpsit,
non commixtionem passus neque divisionem.
A wondrous mystery is declared today,
natures are renewed; God has become man;
that which he was, he remains,
and that which he was not, he has assumed,
suffering neither mixture nor division.