Margam appears to have been an important religious area long before the Normans arrived in 1147. On Margam mountains there are Bronze Age memorial Cairns.
The Romans arrived in Wales in the 1st century A.D and fought bitter battles against the Silurian tribesmen on Margam Mountain.
As the Romans became more powerful, the Celts began to adopt Christianity, which was the official roman religion, they also adopted many roman cultures. When the Romans left in 410 A.D the Celts continued to follow Christianity albeit in a Celtic fashion.
All things roman were considered to be status symbols and signs of authority, consequently local chieftains re-used roman stones as Christian memorials. An excellent example of this is a roman milestone, that was originally erected (about 300 A.D) to honour the Emperor Maximus and re-used in the 6th century as a memorial to a local leader called cantus. There are also squared up-right pillar stones commemorating Pumpeius and Bodvoc both dating back to 6th century. An old legend says that a person reading the inscription on the Bodvoc stone would die within a month.
Fourteen stones from the 9th, 10th, and 11th centuries have been found ion the Margam area. The most famous is the 10th century cross of Conbelin (another local leader) considered to be 1 of the finest Celtic crosses in Britain. Other crosses include those of Ilei, Ilquici, Eunion, and Gruntne. These stones can be seen along with other stones, in the stones museum at Margam. (Formerly one of the oldest church schools in Wales)
Clearly, This shows the importance of Margam as a religious site, long before the coming of the Normans, Furthermore, as many of the crosses were discovered near the site of the present abbey, it is also believed that a Celtic monastery may once have existed at Margam.