Rejoice For You Are True
The Adeptus Ministorum is the body vested with authority to designate how to worship the Emperor. While the Adeptus Terra is nominally a priesthood, they are a priesthood only in that they are chosen to interpret the will of the Emperor and make that happen in the galaxy. The members of the Ministorum are the ones who lead the faithful in service and spread the Emperor’s word.
The Ministorum is led by an Ecclesiarch. In the past, there has been powerful conflict over the leadership of the priesthood, and at times this has led to great disrpution throughout the Imperium. Acolytes, cross reference Dark Heresy p. 245-49 for data on the great War of Faith fought within the Ministorum