The Pantheon

The Pantheon, from Greek Pantheion, meaning "Temple of all the Gods") is a building in Rome which was originally built as a temple to the seven deities of the seven planets in the state religion of Ancient Rome, but which has been a Christian church since the 7th century.
It is the best-preserved of all Roman buildings and the oldest important building in the world with its original roof intact. It has been in continuous use throughout its history. Although the identity of the Pantheon's primary architect remains uncertain, it is largely assigned to Apollodorus of Damascus.

The original Pantheon was built in 27 BC-25 BC under the Roman Empire, during the third consulship of Marcus Vipsanius Agrippa, and his name is inscribed on the portico of the building. The inscription reads M·AGRIPPA·L·F·COS·TERTIUM·FECIT, "Marcus Agrippa, son of Lucius, having been consul three times, built this." It was originally built with adjoining baths and water gardens.

In fact, Agrippa's Pantheon was destroyed along with other buildings in a fire in AD 80, and the current building dates from about 125, during the reign of the Emperor Hadrian, as date-stamps on the bricks reveal. It was totally reconstructed, with the text of the original inscription added to the new facade, a common practice in Hadrian's rebuilding projects all over Rome.
Hadrian was a cosmopolitan emperor who traveled widely in the east and was a great admirer of Greek culture. He seems to have intended the Pantheon, a temple to all the gods, to be a kind of ecumenical or syncretist gesture to the subjects of the Roman Empire who did not worship the old gods of Rome, or who (as was increasingly the case) worshipped them under other names.

(The dome is made of concrete contrary to what my italian source says)