The legend of Saint Agnes is that she was a member of the Roman nobility, raised in a Christian family, who suffered martyrdom at age thirteen during the reign of the emperor Diocletian, on January 21 304.
The prefect Sempronius wished her to marry his son, and on her refusal condemned her to death. Roman law did not permit the execution of virgins, so he ordered her to be raped beforehand, but her honour was miraculously preserved. When led out to die she was tied to a stake, but the faggots would not burn, whereupon the officer in charge of the troops drew his sword and struck off her head.
Saint Agnes is the patron saint of young girls, who, in rural districts, formerly indulged in all sorts of quaint country magic on Saint Agnes' Eve (20th-21st January) with a view to discovering their future husbands. This superstition has been immortalised in Keats's poem, "The Eve of Saint Agnes."
Saint Agnes's bones rest in the church of her name outside the old walls of Rome (hence, Sant' Agnese fuori le mura, "Saint Agnes outside the walls" - on the via Nomentana).
The current church (properly a basilica), as rebuilt by Honorius in the mid-7th century, stands over a 4th century catacomb (one of the most important cemeterial complexes in Rome, with corridors for more than 10 kilometres, only a couple of which accessible). In the fourth century the soft rock had been hollowed out around Saint Agnes's tomb to create a gathering space, probably for her family to observe the anniversary of her death. The visits of her family and friends spread early to others in Rome, and the site became a place of pilgrimage. By the end of the 4th century Costanza, daughter of emperor Constantine, had enlarged the underground area and built a small private temple over it which is now known as the "mausoleo di Santa Costanza" (she was called a saint, even if she was not), while the church of Saint Agnes was then built aside. The floor level of the 7th century church is at the level of the catacomb floor, and the public street entrances are at the level of the 2nd floor gallery.
A later church was built after 1652 on the site of her martyrdom (Sant' Agnese in Agone,) in the Circus of Domitian, now the Piazza Navona, by the important Baroque architect Francesco Borromini. Despite a frequent mistake and the curious assonance, the name of this church is not related at all with the agony of the martyr: "in agone" was the ancient name of piazza Navona ("piazza in agone"), and meant instead (from Greek) "in the site of the competitions", because piazza Navona was by ancient Romans flooded in order to run naval battles in it for fun. From "in agone", the popular use and pronunciation changed the name into "Navona", but other roads around kept the original name (like the Corsia Agonale, a short road that connects with the Palazzo Madama, Italian Senate), as the church did.
At Sant' Agnese fuori le mura on her festival (21st of January) two lambs are specially blessed by the pope after a pontifical high Mass, and their wool is later woven into pallia, ceremonial neck-stoles sent by the popes to newly-elevated archbishops to symbolise their union with the papacy. The church is ruled by a French traditionalist order.
A popular local legend says that every lord mayor of Rome secretly comes to pray to this church, on the third night after his election; effectively, there are not many proofs that new "sindaci" really do so.