The Forum had 4 temples - but the most important was the Temple of Jupiter at the end of the Forum.
The Temple of Apollo, to your left as you look at the Temple of Jupiter, was the most important temple before the arrival of the Romans.
When the Romans arrived, they kept the Temple of Apollo but also financed the Temple of Jupiter, which quickly outshone the Temple of Apollo.
This was typical, as Roman influence progressively superseded that of the Greeks.
Jupiter was the most powerful Roman god. He was god of the skies and thunder and king of the gods.
He was often represented with a thunderbolt and an eagle - symbols which permeated the Roman state and were notably associated with the Roman army.
The temple's main hall was accessible to all, while the inner chamber was only accessible to priests and would store religious offerings and the treasures of the city.