Celt's Druidic Lore isn't meant to be used for lots of faith forever. The point is to ensure they get first pick of all beliefs. No other civ can start the game gaining faith from turn 1, ensuring first Pantheon even on higher difficulties. Their unit produces faith on kill too, so you should have first Religion and first Enhanced Religion. This first pick can be very important.
After that, the faith from forests and kills isn't as important, but the Celts still have an amazing building to use, so they've always got pretty nice bonuses. Feel free to upgrade forests later on if you need to.