This hymn is of unknown authorship and date. It is for the Feast of the Sacred Heart of Jesus; for which Feast in some editions of the Roman Breviary later than 1735 there are two distinct offices with different hymns; the day of observance being that following the Octave of Corpus Christi (viz. Friday before the 3rd Sunday after Whitsunday). Auctor beate saeculi is the hymn at second Vespers in the first office when the Feast is kept on its own day, and with the rank of a greater double; and at both Vespers when the Feast is transferred, or kept with the rank of a double of the first or second class, the reason being that in the former case the first Vespers are superseded by the second Vespers of the Octave of Corpus Christi. In England the first office is appointed to be said on the Sunday after the Octave of Corpus Christi, with the rank of a double of the second class; religious orders, as a rule, observing it on the Friday succeeding that Octave, thus the hymn occurs at both Vespers.
Contrasting flutes with a brief appearance by flute 4 and Nazard