Sacred is an attribute units and commanders can have which allows them to be blessed and therefore gain the blessing bonuses of your Pretender, alongside the +1 morale which is automatic for any bless.

Usually if something is referred to as a sacred, it means a sacred unit, which are usually a cut above other units without a bless and require more gold to recruit, but with one which is carefully chosen to complement their stats they can be devastating. Any commander that can cast holy spells is also considered sacred by default, and any sacred commander can build a temple in a province.

In addition, sacred units only require half the upkeep cost of an equally priced non-sacred unit.