One characteristic of the multi-cultural Roman religion is the adoption of other Gods as needed. At other times, certain aspects of various Gods would be combined into one God or foreign Gods were correlated with Roman ones. Influenced by the Etruscans, the original Triad of Gods overseeing Roman life changed.
The Etruscans had their own Triad of Gods: Tinia, Uni, and Menrva. Their Supreme God was Tinia (Tin). To them, He was Father Time since He governed time as well as the skies. Tinia was usually depicted hurling lightening bolts. (Saturn, not Jupiter, was Father Time for the Romans.)
Meanwhile, His Wife and Sister, Uni was the supreme Goddess of the Etruscans. She protected them and their rulers. In addition, Uni governed all aspects of Etruscan womanhood from the wedding to nursing children. (Juno, as the wife of the Roman Jupiter, was equated to Uni.)
Tinia and Uni’s daughter was Menrva, the Goddess of War, Wisdom, and the Arts. She was usually depicted with a helmet and spear. Since She governed the weather, Menrva also threw lightening bolts. (The Roman Minerva was originally this Etruscan Goddess.)
Capitoline Triad (Jupiter, Juno, Minerva)
The Capitoline Triad oversaw the affairs of Rome and her people. Jupiter Optimus Maximus (Brightest and Best) protected the state. In addition, Juno Regina was the guardian of Rome. Together, They guided the affairs of the Roman people. Meanwhile, Minerva was the Patron of Doctors and the Arts.
After the wars with the Sabines, King Tarquin Priscus asked the Deities of a shrine on Capitoline Hill to move so that he could build a temple to Jupiter Optimus Maximus. In exchange for their leaving, he promised Them a new temple elsewhere (exauguration). All the Gods did except for Terminus, the God of Boundaries. The Romans regarded this as a good omen. The new temple was dedicated to Jupiter, Juno Regina, and Minerva (the Capitoline Triad). (However, a part of it remained a shrine to Terminus.) On the Ides of September, the praetor maximus (head magistrate) would drive a nail into the wall of the temple (cella Iovis). This was to ward off the plague for another year.
The main temple for the Capitoline Triad had three rooms with each God having their own space. Jupiter Optimus Maximus occupied the center cella (room), with Juno Regina on the left and Minerva on the right. Although the temple was build during the time of Roman Kings, it was dedicated by the first Consul of the Roman Republic.
Known as The Shining Father (“Dies Pater”), Jupiter, according to the Romans, is the Ruler of the Cosmos. Jupiter Optimus Maximus (IOM) is the Supreme Roman God. As the Lord of the Sky, He makes his will known through thunder and lightning. Any piece of land struck by lightning belongs to Him alone.
The Romans looked to Jupiter as the Protector of Rome and its laws. They saw Him in many aspects of governance. As a member of both the Archaic and Capitoline Triads, Jupiter Optimus Maximus oversaw Roman affairs. As Jupiter Lapis, He presided over solemn oaths. Meanwhile, Jupiter Feretrius presided over treaties and just wars. Jupiter Stator encouraged the Romans to stand their ground against the Sabines and later the Samnites.
Jupiter Pistor appeared to the Romans during the siege of their Capitol by the Gauls. He told them to hurl bread at the attacking Gauls. Believing that the Romans had ample supplies, they decided to leave. The Gauls ended their siege not knowing that the Romans had thrown the last of the food stores at them.
In addition, many of Jupiter’s titles allude to his control of the weather. The Romans delineated his many forms of thunder and lightning in their names for Him. Jupiter Elicius regulated the rainfall, while Jupiter Tonans nearly struck the Emperor Augustus with lightning.
The oldest temple for Jupiter was Jupiter Feretrius, founded by Romulus. This temple was a repository of ritual implements for dedicating treaties. To declare war, the fetialis (priest-diplomat of Jupiter) would hurl a spear from the temple into enemy territory. To solemnize a treaty with foreign governments, the fetialis, using the lapis silex (flint) of Jupiter Feretrius, sacrificed a pig.