Depends on what religion you are speaking of. The ancient Celts were polygamists, believed in many gods. Before the Medeival era (500 ad-1300 ad something like that), Catholic conversion among the Celts began to take place and the Celts became a generally catholic race. after the great schism between Catholicism and Protestantism Many became Protestant and Killed off many of the Catholics
This question requires an extensive and complex answer. First of all, the Celts were a group of different tribal societies that lived in Europe from about 1200 B.C.E. through the Middle Ages. Over the many centuries, as the religions of Rome had a greater effect on the Celts, the religious beliefs of the Celts ranged from polytheistic to Christian.
saw spirits everywhere ;rivers tress stones pond fire.
Druidism
What I think the religion of the Celtic cross came from is probably the Pagan Druidic religion.
The ancient Vikings were of the Nordic religion, a folk religion with Thor as the head Guy and a bunch of other gods.
Yes, Wicca does exist. Wicca is a pagan revivalist religion that incorporates many ancient Celtic traditions with other pagan pantheons.
Boudicca was a Celtic queen and warrior queen of the Iceni tribe in ancient Britain. She likely practiced Celtic pagan beliefs, which involved reverence for nature, rituals, and deities associated with the natural world.
Celtic religions were practiced until the various Celtic peoples were Christianised in the early centuries A.D.
Duidism
Celtic religion started before Jesus had even come, they have been around forever, but the longer they go on the more they change and are forgotten.
The "Celts" of the English Isles were known for their Druid religion ( a pagan or Wicca like religion) but there ws also a large Christian grouping. The Celtic Cross was designed by these early Celtic Christians.
A Druid
They practiced the ancient Egyptian religion.
HINDUISM - The World's Ancient RELIGION.
Napoleon