What would the society of the Middle Ages be like in a celtic Europe?

[dk900]

Imagine that Rome was a Celtic city that managed to develop and conquer the meditarranean.
Over time this Empire suffers the same fate that in our timeline, is divided and the western part falls.
What would a Middle Ages be like in this situation?
How would society be? The government systems? The religion?
I know there are many butterflies, but it seems very interesting to me and I would like you to think how you think it would be.

For example, religion:
In this world there is no Christianity and probably neither Islam.
In this situation there are several possibilities:
- Europe remains pagan, conserving Celtic traditions and other native religions.
-Eventually someone would "reform" Celtic paganism into an organized religion.
-New religions born of syncretism would replace paganism, like Sol Invictus in our timeline
-The eastern religions (Zoroastrianism, Manicheism etc ...) would end win over European paganism.
- Philosophies like such as Neoplatonism, if they existed in this timeline, would become state religions such as Taoism or Confucianism in China

 

[CountPeter]

There are some big issues with your assumptions.

Re Christianity: if there is no Christianity, there is 100% no chance of there being Islam. Islam came after Christianity, and even has Jesus as an important figure.
Aling this line, Manichaeism, another post-christian abrahamic religion would also be butterflied away entirely.

Re what the empire is like, it being a celtic replacement of rome doesn't tell us anything. Celtic peoples spanned a large area of Europe with differing traditions etc. For all we know, alt-celt rome could just be exactly the same.