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:

• Holy Word (stuns)
• Apostasy (charms; only for Sceleria and Lemuria)
• Halt Heretic (Awe for Sacreds)
• The Unholy Weapons bless
• The "Bane of Heresy" effect of certain magic weapons, such as the Black Halberd and the God-Slayer Spear

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 to a lack of Commander Points or Resources when you recruit them) requires a Holy Point, though some require two. Your per-turn number 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.