Like the people living in Britain prior to 800 BC, the Celts had no written language. This makes it obviously very difficult for us to develop a clear understanding of their religious beliefs. What we do have is the writings of the Romans who conquered them, which in itself is problematic because, when victors write history, invariably they are writing from a position of their own magnificence, and by definition are seeking to downplay and barbarize the achievements and culture of those they conquered.
Scholars do think there was a basic religious homogeneity among the Celts, with significant regional differences, especially since they were spread out across Europe from Czechoslovakia to Ireland. Like the Romans who came to Britain after them, the Celts were polytheists, believing in gods and goddesses, upwards of 200 of them. For example, the god Lleu in Welsh mythology, a warrior God and ultimately the King of Gwynedd, is associated with the Gaulish Lugus and the Irish Lugh. For the Irish, the pantheon of gods is known as the Tuatha de Dannan. Among the Welsh, they are the Mabinogi.
The Roman conquest was a turning point in Britain because the Romans viewed the Celtic religion as subversive and encouraging rebellion, and they directed significant resources to wiping out not only the druids, who were the Celtic priests, but all sacred sites. These actions culminated in the year 60 with a Roman attack on Anglesey, as recorded by the writer, Tacitus:
“He [Suetonius Paulinus] prepared accordingly to attack the island of Mona … [in order to cross the Menai Strait he] constructed a flotilla of boats with flat bottoms. By this method the infantry crossed…
On the beach stood the adverse array, a serried mass of arms and men, with women flitting between the ranks. In the style of Furies, in robes of deathly black and with dishevelled hair, they brandished their torches; while a circle of Druids, lifting their hands to heaven and showering imprecations, struck the[Roman] troops with such an awe at the extraordinary spectacle that, as though their limbs were paralysed, they exposed their bodies to wounds without an attempt at movement.
Then, reassured by their general, and inciting each other never to flinch before a band of females and fanatics, [the Roman forces] charged behind the standards, cut down all who met them, and enveloped the enemy in his own flames.”
Tacitus goes on to say: “The next step was to install a garrison among the conquered population, and to demolish the groves consecrated to their savage cults: for [the druids] considered it a pious duty to slake the altars with captive blood and to consult their deities by means of human entrails.”
As I mentioned at the start, it is an open question as to how much we believe of what this one Roman wrote and how much we reject as writings of a victor denigrating the religion of the people they’ve just destroyed. Regardless, the Britons’ religion syncretized with the Roman pantheon after the conquest, and then again with Christianity after 300 AD.