The Romans were generally quite tolerant of foreign religions but made an exception in the case of Celtic Druidism. The Roman governor of Britain in AD 59 was one Suetonius Paulinus. He became aware that, especially in Wales, resistance to Roman rule was fired by the Druids. The Druids seem to have had a pervasive influence in Britain and the governor felt obliged to invade the Isle of Anglesey which was especially sacred to the Druids. The island was attacked, Druids were killed and the sacred groves were destroyed. The Romans complained that the cult was responsible for human sacrifices but it may be that they simply resented the power that the Druids exercised. Soon after Druidism was forbidden in Britain, the Romans tried to introduce the cult of emperor worship but this (and its associated financial demands) was much resented.
Caesar, writing of the Celts of Gaul, says that they revered the Roman gods. He tells us that in his day the Gauls worshipped the gods Mercury, Apollo, Mars, Jupiter and Minerva. Mercury was considered to be the most important of these and images of him were plentiful.
We know, however, that the Celts worshipped their own local gods alongside the Roman imports. They worshipped a goddess called Epona and a god of war called Esus as well as many minor gods and godesses associated with natural features like rivers and springs. A stone relief at Rhiems in France depicts the horned god Cernunnos wearing a neck torque. Temples were not generally thought necessary but sometimes appeared at particularly important sites. The wild boar seems to have had a religious significance and was sometimes used in the form of a war helmet crest.
The Druids taught that the soul does not die. The soul of a man who
died in battle would pass to another body and Caesar thought that this
belief partly explained the bravery of the Celts in war. When they were
not busy learning their verses, the Druids would "hold long discussions
about the heavenly bodies and their movements, the size of the universe
and of the earth, the physical constitution of the world, and the power
and properties of the gods ...". They would instruct pupils in these subjects.