Located in the heart of Central Rome, the Pantheon was built in ancient Roman times as a temple to all of the Gods. It was built in the early second century. Of course, it was not long that it actually housed worship to the Olympian Gods, as Christianity became the state religion in the early fourth century.
When Olympian worship effectively ended, the Pantheon essentially stood idle for a couple hundred years, before being used as a church again. Indeed this pagan building has been a Christian (Catholic) church for most of its history.
It's also famous in the history of the Renaissance because builders of two famous, domes, Brunelleschi (who built the dome in the Duomo in Florence) and Michelangelo (who designed the dome of St. Peters in the Vatican City) both studied the dome.
And the dome is indeed the overwhelming feature of the place, large, strangely graceful despite being made of essentially ancient concrete, it rises large and far above you. We were there in mid afternoon, and light streamed through the dome down into the building itself. It is open to the world and yes, when it rains, it just rains in.
Columns remain from the original construction, in remarkably good condition (the entire place is in incredible condition for a building that is close to 1900 years old). Most of the interior decoration, particularly sculpture, is Christian in nature, although you can see where the statues of the Olympian Gods would have been.
When we visited they were saying mass. This does not preclude visiting, though it limits where you can walk significantly. One needs to remember that this is indeed a church. As we were visiting as pilgrims, it did feel a little intrusive to enter as viewers, but while we were in there an unseen choir burst into a beautiful rendition of Mozart's "Ave Verum Corpus" which resonated off the stone and the dome and filled the place with great spiritual beauty. With the sunlight filtering through the ceiling, the candles flickering soundlessly and the heavenly statuses around us, it felt like a true moment of worship.