In ancient Roman religion, Sancus (also known as Sangus or Semo Sancus) was a god of trust (), honesty, and oaths. His cult, one of the most ancient amongst the Romans, probably derived from Umbrian influences. Cato and Silius Italicus wrote that Sancus was a Sabine god and father of the eponymous Sabine hero Sabus. He is thus sometimes considered a founder-deity.
Oaths
Sancus was the god who protected oaths of marriage, hospitality, law, commerce, and particularly formal contracts. Some of the oaths said at the moment of signing a contract – or other important civil promissory acts – named Sancus as guarantor, and called on him to protect and guard over the honour and integrity of the signatories' pledges.
Etymology
The place-name is related to the theonym, through the proper name .
Sancus derives from a Latin/Umbrian/Sabine source (compare Umbrian sacra/ sakra "sacred", and the Umbrian theonym Sansi/Saçi), and is connected to Latin ('to hallow') and hence sanctus ('hallowed') and English saint, sacred, sanctuary, sanctity and sanction.
Outside Italic, Hittite has a number of words such as saklai "rites" and sankunnis "priest" that seem to be clear cognates, suggesting an Indo-European root *sank-/*sak- "to sanctify". It is possible, however, that the former was a loan from Akkadian sakku(m) 'cultic rites,' and the latter may be a loan word from Sumerian sanga "administrative priest".
Woodard has interpreted Sancus as the Roman equivalent of Vedic god Indra (whose name is likely related to Sanskrit indu "rain drop"), who has to rely on the help of the Maruts, in his view corresponding to the twelfth Roman of the carmen Arvale, in his task of killing the dragon Vrtra thus freeing the waters and averting draught. He traces the etymology of Semo to PIE stem root * bearing the meanings of 'to pour', 'ladle', 'flow', 'drop' related to rain and sowing. (See more below at "Mars and Semo")
Worship
The temple dedicated to Sancus stood on the Quirinal Hill, under the name .
Dionysius of Halicarnassus writes that the worship of Semo Sancus was imported into Rome at a very early time by the Sabines who occupied the Quirinal Hill. According to tradition his cult was said to have been introduced by the Sabines and perhaps king Titus Tatius dedicated a small shrine. The actual construction of the temple is generally ascribed to Lucius Tarquinius Superbus, although it was dedicated by Spurius Postumius on 5 June 466 BCE.
Sancus was considered the son of Jupiter, an opinion recorded by Varro and attributed to his teacher Aelius Stilo. He was the god of heavenly light, the avenger of dishonesty, the upholder of truth and good faith, the sanctifier of agreements. Hence his identification with Hercules, who was likewise the guardian of the sanctity of oaths. His festival day occurred on the of June (5 June).
Shrine on Quirinal
The shrine on the Quirinal was described by 19th century archeologist R.A. Lanciani. It was located near the of the Servian walls, not far from the modern church of San Silvestro al Quirinale, precisely on the . It was described by classical writers as having no roof so as oaths could be taken under the sky.
It had a chapel containing relics of the regal period: A bronze statue of Tanaquil or Gaia Caecilia, her belt containing remedies that people came to collect, her distaff, spindle, and slippers, and after the capture of Privernum in 329 BCE, brass medallions or bronze wheels (discs) made of the money confiscated from Vitruvius Vaccus.
Dionysius of Halicarnassus records that the treaty between Rome and Gabii was preserved in this temple. This treaty was perhaps the first international treaty to be recorded and preserved in written form in ancient Rome. It was written on the skin of the ox sacrificed to the god upon its agreement and fixed onto a wooden frame or a shield.
According to Lanciani the foundations of the temple were discovered in March 1881, under what was formerly the convent of San Silvestro al Quirinale (or ), later the headquarters of the (former) Royal Engineers. Lanciani relates the monument was a parallelogram in shape, thirty-five feet long by nineteen wide, with walls of travertine and decorations in white marble. It was surrounded by votive altars and the pedestal of statues. In Latin literature it is sometimes called aedes, sometimes sacellum, this last appellation probably connected to the fact it was a sacred space in the open air. Platner & Ashby (1929) though write that its foundations had already been detected in the 16th century.