4000 B.C.: The warring Celtic Tribes come together under a single cheiftain, founding the settlement of Edinburgh on the wooded banks of the River Celtica.
The Celts' civ power grants me bonus Faith in my cities for every adjacent forest tile, so this starting location was very fortunate. I'll have an early lead on founding a religion, especially once I research Bronze Working and can start farming barbarians with my Pictish Warriors.
3875 B.C.: The Celts develop an Honor-based warrior culture.
I go into Honor as my first Policy- sort of "talent trees" that will give my civ different bonuses. Taking this one early gives me a bonus when fighting randomly-spawning barbarians, which will further help my Pictish Warriors gain Faith.
3800 B.C.: The disparate superstitions of the Celtic people are unified into a single pantheon, led by a powerful messenger god.
Being the first to found a Pantheon is a big deal, because it makes the Fatih cost for all the other civs to do so go up. I choose to follow the Messenger of the Gods, which grants me bonus Science for the rest of the game in any city with a trade route. It's the only Science-based Founder Belief, and plays perfectly into my Religion into Science transition plan.