Dius Fidius — the Divine of the Faith — was an old Italic god of the sworn oath, the divinity by whom Romans swore their most binding oaths. He was identified with the Sabine god Semo Sancus (sometimes Sancus Dius Fidius, in the long form), and was perhaps the same god whom the Umbrians invoked as Sancius. His temple on the Quirinal Hill, traditionally founded by the king Tarquinius Superbus and dedicated in the early Republic, was one of the very oldest in Rome, and contained a famous statue of Gaia Caecilia, the wife of Tarquinius, with a spindle and distaff — the matron-image preserved through every later renovation.

The peculiar feature of swearing by Dius Fidius was that the oath had to be taken in the open air, beneath the sky, for the god was a sky-god (the dius element being cognate with Jupiter, deus, dies — the bright sky). Even his temple had a hole in the roof, that the sworn oath might pass directly to the god above. This requirement of open-air swearing distinguished Dius Fidius's oaths from those by Jupiter Lapis (sworn upon a stone) and by Hercules (sworn in the marketplace).

By the imperial period, Dius Fidius had been largely assimilated to Jupiter or to Hercules (whose Greek epithet pistios meant the same thing — "of the faith"), and his independent cult had faded. But his temple on the Quirinal continued to stand, and inscriptions to him as Sancus Sanctus Semo Deus Fidius are found through the second century. The sect of Christians whom the apologist Justin Martyr defended in the second century was charged, in Justin's report, with worshipping Simon Magus the heresiarch as a god — a charge based on a confused reading of an inscription to "Semoni Sanco Deo Fidio" on the Tiber Island. The misreading of Semo as Simon is preserved in Justin's First Apology and gave rise to a small literary controversy that stretched for centuries, until the rediscovery of the actual inscription in the seventeenth century settled the matter.