St. Agnes

Today is the birthday of a virgin; let us imitate her purity. It is the birthday of a martyr; let us offer ourselves in sacrifice. It is the birthday of Saint Agnes, who is said to have suffered martyrdom at the age of twelve. The cruelty that did not spare her youth shows all the more clearly the power of faith in finding one so young to bear it witness.
~from a treatise "On Virgins" by Saint Ambrose, bishop, in the Roman breviary.

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Of all the virgin martyrs of Rome none was held in such high honour by the primitive church, since the fourth century, as St. Agnes. In the ancient Roman calendar of the feasts of the martyrs (Depositio Martyrum), incorporated into the collection of Furius Dionysius Philocalus, dating from 354 and often reprinted, e.g. in Ruinart [Acta Sincera Martyrum (ed. Ratisbon, 1859), 63 sqq.], her feast is assigned to 21 January, to which is added a detail as to the name of the road (Via Nomentana) near which her grave was located. The earliest sacramentaries give the same date for her feast, and it is on this day that the Latin Church even now keeps her memory sacred.

Since the close of the fourth century the Fathers of the Church and Christian poets have sung her praises and extolled her virginity and heroism under torture. It is clear, however, from the diversity in the earliest accounts that there was extant at the end of the fourth century no accurate and reliable narrative, at least in writing, concerning the details of her martyrdom. On one point only is there mutual agreement, viz., the youth of the Christian heroine. St. Ambrose gives her age as twelve (De Virginibus, I, 2; P.L., XVI, 200-202: Haec duodecim annorum martyrium fecisse traditur), St. Augustine as thirteen (Agnes puella tredecim annorum; Sermo cclxxiii, 6, P.L., XXXVIII, 1251), which harmonizes well with the words of Prudentius: Aiunt jugali vix habilem toro (Peristephanon, Hymn xiv, 10 in Ruinart, Act. Sinc., ed cit. 486). Damasus depicts her as hastening to martyrdom from the lap of her mother or nurse (Nutricis gremium subito liquisse puella; in St. Agneten, 3, ed. Ihm, Damasi epigrammata, Leipzig, 1895, 43, n. 40). We have no reason whatever for doubting this tradition. It indeed explains very well the renown of the youthful martyr.

The Amo Christum is an ancient Responsory for the feast of Saint Agnes, also used at religious professions:

I love Christ, into whose chamber I shall enter, whose Mother is a virgin, whose Father knows not woman, whose music and melody are sweet to my ears. When I love Him, I remain chaste; when I touch Him, I remain pure; when I possess Him, I remain a virgin. With His ring my Lord Jesus Christ has betrothed me and He has adorned me with the bridal crown.