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 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.