The guy who played for Celts left the game, because he has had periodical game crashes on his turn. So we are looking for another player to sub him. But it does not mean that the sub will also have the same CTDs when loading the save file.
Celts have awesome early UU, the pictish warrior, which so much suits for the tactics of "pillage and heal".
There are four religious biased civs in the game (Arabia, Maya, Celts and Byzantium). So the leader of celts will have to struggle for religious followers against religions of competitors.
At the moment Celts have been ruled by AI for 2 turns. You should give them a try