The Isis Temple in Moguntiacum (Mainz)
Moguntiacum has been the location of a legionary and an auxiliary fort since 16 BC when Augustus' son-in-law Drusus started the first expedition into Germania on the other side of the Rhine. The place, as usual, attracted civilian settlements, and by the end of the 1st century AD had become the capital of the province Germania Superior. By that time, the forts had been rebuilt in stone and the civilian settlements had developed structures of a Roman town with a forum, a theatre, baths, temples, and a harbour. Moguntiacum remained a major Roman place until the Migration Period in the 4th century.

So whenever someone digs a pit, the County Office for Historic Preservation (Landesamt für Denkmalpflege) will be involved. They also had a look when the earth was scooped out for fonduations of a new shopping mall in 1999. Since the place lay over the road that once led to the Roman bridge, they expected to find some remains of artisan's workshops and small stores usually located in such places. The foundations of a temple five metres under todays level came as a surprise. Even more, under the temple was a Celtic Hallstatt burial from about 680-650 BC. A seventeen months rescue dig took place to save all moveable artifacts of both the temple and the grave.

View of the Taberna archaeologica with its 'starlit sky'
At first, it was planned to document the foundations and then cover the place up again, but the inhabitants of Mainz wanted nothing of that. They started a public campaign and convinced the local government to preserve the temple. Since the remains were in the way of the planned underground parking, they had to be moved a few metres along the 'road', an endeavour that cost about 3.5 million Euro, funded by the Town of Mainz and the County of Rhineland-Palatinate.

Since 2003, there is a museum on cellar level of the shopping mall 'Römerpassage', the Taberna archaeologica, that can be reached from the mall (admission is free). It shows the remains of the temple and the finds made on the site in a modern multimedia display with glass walkways over the foundations, voiceover of a Roman who visits the temple, a film projection at one wall and changing illumination that makes for a nice atmosphere and difficult photographing, because I could not figure out a pattern to the lights. The place is usually rather dark but sometimes the sacred fires flare up, so to speak, and that's the best moment to get shots of the foundations. There are also some display cases with the finds, including the bones and jewellery of that Celtic lady.

Some of the temple foundations
The temple is dedicated to Isis Panthea and Mater Magna / Cybele. How did goddesses from the Nile and Asia Minor end up at the Rhine? Well, the Graeco-Roman pantheon rarely met a god or goddess they didn't like. The orgiastic cult of Cybele, a goddess of the earth, fertility and wild animals, had found its way to Rome already in the 3rd century BC (though the self-castration of her priests was forbidden) and the Egyptian gods were popular with the Greek Ptolemaic dynasty in Alexandria. The Isis cult was forbidden in Rome during the times of Augustus and Tiberius because of the recent war with Egypt, but it became popular again under Caligula.

Like the Mithras cult, the worship of Isis is a mystery cult, open only to initiates (but contrary to the exclusively male Mithras cult, both men and women could join). Those cults were henotheistic, that is they merged several gods into one, like Isis Panthea (the All-Goddess) or Mater Magna (the Great Mother) but accepted the existence of other gods. Thus a Roman who worshipped Mithras or Isis had no problems in partaking in the official cult of Jupiter Optimus Maximus and the Deified Emperors as well. The official cult was integrative for the empire and by refusing to join it, the Christians put themselves outside the empire while still living within its boundaries; one of the reasons they were not officially accepted for a long time.

Foundations at the moment the 'fires' are on
There is proof for the existence of Isis temples at the Rhine since the 2nd century AD, but the one in Moguntiacum is interesting because it dates earlier: to the reign of Vespasian, AD 69 - 79. A building inscription mentions him: "Primigenius [unreadable word], of the Imperator Vespasianus Augustus the procurator of the finances, by order for the Great Mother, has erected [=this building]." (yep, that's Latin syntax for you, lol). Means, Vespasian's minister of the finances, one Primigenius, has built the temple dedicated to the Great Mother, and done so by imperial order.

We further have two tabulae ansatae, stone tablets with dovetail handles, that read "pro salvte avgvstorvm spqr exercitvs isidi panthaea (the other one has: matri magnae) ..." (= for the benefit of the emperors, the senate and people of Rome and the army, to Isis Panthea / Mater Magna ...), followed by the names of the dedicants, Imperial freedmen. The formulas used are official ones, not something a private worshipper would have written. The official character of the building can also be proven by a number of tiles with legionary inscription that have been found, which means that the legions (fe. the XXII Primigenia, stationed in Mainz at the time of Vespasian) were not only involved in the construction work but also used army material.

The dedication stones (the tabulae ansatae are to the left)
Vespasian had a soft spot for the Egyptian gods. Before claiming the postion of emperor, he visited the Serapis temple in Alexandria and was told by the god he'd be successful. According to Sueton, Serapis even worked a miracle on Vespasian's behalf. Serapis is another of those syncretistic gods, uniting Zeus, Osiris and Apis, the sacred bull; he was the god of the Nile flood (fertility), healing and oracles, but also of death. He is the husband of Isis who became the protective goddess of the Flavian dynasty like Venus had been for the Julio-Claudian emperors. There must have been a temple of Jupiter and/or the Capitolinian Trias in Mainz as well, but it has not been found.

The commission of the Isis temple fell in a time of political change and could have been meant as some sort of symbolic act. Vespasian had won the strife for the imperial purple. The civilian settlements of Moguntiacum had been destroyed during the Batavian rebellion, together with the fort at Vetera (Xanten) and other places, so there was a lot of building going on along the middle and lower Rhine. Moreover, the Rhine legions had to be restructured; two legions had been almost destroyed when Vetera fell (V Alaudae, XV Primigenia), thoase that sided with the rebels were disbanded (XVI Gallica) or partly disbanded and merged with another legion (I Germanica, merged with VII Gemina).

Overview with a projection of an Isis statue
The Hallstatt burial mounds must still have been visible at the time the temple was built, thus the part of the area along the road between the fort on the Kästrich hill and the Rhine bridge may have been considered sacred ground. It could explain the lack of the usual artisan workshops and other hovels that gathered along such roads. One of the graves was then deliberately used as centre of the temple.

The Isis / Mater Magna temple doesn't follow the style of Gallo-Roman arcade temples (Umgangstempel another untranslatable word) one would expect north of the Alpes. Arcade temples have one rectangular central room, the cella, and a procession walk around it, often pillared and open to the outside, but roofed in; the cella was usually higher than the colonnade.

The Isis temple is a walled area with different buildings instead, which moreover have been altered significantly during the 200 years the temple was in use. When town walls were added AD 250, the temple was included in the protected area.

The temple with clouds of incense smoke
The first temple area consisted of two small rectangular temples, a latrine (now, that's a luxury you won't find in modern churches) and two simple half-timbered single room houses equipped with hearths and wells. They may have served as meeting rooms.

Changes in the 2nd century enlarged the temple area to 16x16 metres. The main building consisted of two larger rooms along a central axis, surrounded by several smaller ones, including a well room - water played an important role in the Isis worship. The single door was located opposite the well room. Outside the central building, in the inner yard, are three massive stone bases that may have held altars. There are also traces of several fire places with remains of burnt offerings, and deposit pits.

Detail of the outer wall foundation with stucco decoration
The buildings were half timbered with stone foundations (and those luckily remain until today). The loam covered willow tracery between the beams had a layer of roughcast with decorative frescoes. Scores of bits of coloured roughcast and stucco have been found, including a larger fragment showing Anubis with a staff and a palm tree. The floor seems to have been but hardened clay; the buildings were roofed in with tiles and shingles. Makes me wonder how shingles fared in the German rain; they may have needed regular replacing.

The cult of Isis and Mater Magna in Moguntiacum obviously was no longer active towards the end of the 3rd century though we don't know the reason. But all dateable finds point to the 1st and 2nd centuries AD, and during that time the temple was a lively place of worship.

Another overview
Here's a guest post by Stephanie Dray, who tells us more about the Isis worship. A post about the finds in the Isis Temple can be found here.

Sources:
The extensive German Wikipedia article about the Isis temple in Mainz which is based on the guidebook that's currently out of print.
Roland Gschößl, Im Schmelztiegel der Religionen - Göttertausch bei Kelten, Römern und Germanen. Mainz 2006  ¶ January 23, 2011