Celtic warriors used to cut off the heads of their enemies in battle and display them as trophies. They mounted heads in doorposts and hung them from their belts. This might seem barbaric to us, but to the Celt the seat of spiritual power was the head, so by taking the head of a foe they were appropriating that power for themselves. It was a kind of bloody religious observance.
The Celts were a very spiritual people. They had many gods and goddesses in which they revered nature and the world around them. The Celtic religious system operated in a way much like the Romans. The only significant difference is that while the Romans and Greeks thought their deities were immortal, the Celts believed nothing could escape the grasp of reality and death. This is evident especially with Cuchulainn, who is a very well known hero who dies on the battlefield in the myth of" The Cattle-Raid of Cualnge”.
The Celts had lots of large festivals held in honor of certain gods or goddesses whom they felt deserved special recognition for their role in life.
Celtic priests were called druids, and their religion, druidism. Little is known of the druids because their rites were never written down. Apparently their gods were similar to those of other early peoples. The druids of Gaul were both judges and priests who sacrificed criminals to their gods.
Druids were also judges, mediators and political advisors. A person who in the future becam druid was usually taken from noble families and was taught from the child hood. To eventually become druid, a person needed 20 years.
Druids didn't believed in heaven or hell, but they preached an automatic reincarnation on Earth, regardless of one's deeds in life.