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A Sacred unit is able to be Blessed.
Aside from that, Sacred units require half as much Upkeep, but they typically cost more to hire than non-sacred units of the same skill set. They're also vulnerable to certain things:
Last-but-not-least, sacred units require Holy Points to hire. Every turn needed to recruit them (such as when the process takes another turn, due lacking Commander Points or Resources when you recruit them), they require a Holy Point, though some require two. Your per-turn of Holy Points for each fort is equal to your Pretender God's Dominion strength, plus 1 for every five temples you have; Ind and Piconye get 1 Holy Point for each Temple they have, however.