The Pantheon was originally built from 27-25 BC. It was built by the magistrate Marcus Agrippa to commemorate the victory of Actium. This was a victory over Antony and Cleopatra. In 80 AD the original temple burned down.
Hadrian reconstructed it in 125 AD, and dedicated it to every known god. Septimus Severus and Caracalla maintained and restored the Pantheon.
During its use, statues of gods lined the walls, sacrifices were made in the center, and smoke rose out through the oculus. When Christianity replaced paganism, the temple fell out of use for awhile.
Pope Boniface IV eventually consecrates the temple as a Christian church. The church was dedicated to Mary and all martyrs, so again the Pantheon is not for specific gods/people.
In 667, the Pantheon was stripped of everything of value inside, including the golden roof tiles. It was robbed and restored multiple times.
Michelangelo studied at the Pantheon before he designed St. Peter's, and Bernini stripped the Pantheon's bronze for a project in St. Peter's. The bronze doors were restored in 1563.
Picture from: http://www.davidmixner.com/images/2008/06/07/pantheon.jpg
