From The Art and Popular Culture Encyclopedia
Jupiter may have originated as a sky-god, associated primarily with wine festivals and the sacred oak on the Capitol. If so, he developed a twofold character. He received the spolia opima and became a god of war; as Stator he made armies stand firm, and as Victor he gave them victory. As the sky-god, he was the first resort as a divine witness to oaths.
Jupiter's primary sacred animal is the eagle, which held precedence over other birds in the taking of auspices.
Jupiter was the central deity of the early Capitoline Triad of the Roman state religion comprising Jupiter, Mars and Quirinus, who each possessed some measure of the divine characteristics essential to Rome's agricultural economy, social organisation and success in war. He retained this position as senior deity among the later Capitoline Triad of Jupiter, Juno and Minerva.
In the Greek-influenced tradition, Jupiter was the brother of Neptune and Pluto. Each presided over one of the three realms of the universe: sky, sea, and the underworld. Jupiter remained Rome's chief official deity throughout the Republican and Imperial eras, until displaced by the religious hegemony of Christianity.