Are there no believers of Divine Spirits within the Empire?
Of course, there are. They're quite common, in fact. It's just that the Empire Council stipulates that all faiths must be registered, all Church activities must be approved by the Judicial Court and the local Town Mage, and...
No collecting tithing, and no selling Absolution Certificates!
The end of the Old Empire coincided with the advent of the Silent Age of the Gods. When people discovered the connection between the Divine Kingdoms and the mortal world wasn't as strong as before, the Divine Teachings took a major blow. The Church of Light and Order, which had once dominated the continent, suffered the greatest losses:
