If your objective is the prevention of heresy, it's worth while being sure about the truths you seek to protect. This work looks at the core faith of the Imperium: belief in the immortal God-Emperor. This is led by the Adeptus Ministorum, or Ecclesiarchy - the priests who minister to the faithful and spread the word across the galaxy. Whether you intend for the characters to interact with the faithful and their religious leaders or want to run a game in which they take on such positions themselves, there's a wealth of information here to get you started.
The first chapter, A Million Worlds, One Emperor, sets the scene. Starting with the basics, it discusses the Imperial Creed, looking at how it developed and how the faith operates today in the 41st century with particular attention to how it is organised in the Calixis Sector. In essence, it's quite reminiscent of the role played in mediaeval Europe by the Catholic church, not just providing a belief system but serving as a controlling force establishing order.
Next, Chapter 2: Paths of the Righteous provides the game mechanical information you'll need to run characters who are part of the Ecclesiarchy - everything from new homeworlds and backgrounds to new ranks to reflect the specialist careers available. There's more too, the different slants that various people take on the common faith and how they have formed factions and cults that each put their own spin on commonly-held beliefs.
Then Chapter 3: Brides of the Emperor presents the Adepta Sororitas, or Sisters of Battle. Some may have felt a lack of good opportunities for female characters so far in this game, here is a chance for them to come into their own. Here you can read about what they do, and find all the rules necessary to create a Sister character.
In Chapter 4: Faith and Fury there are rules and game mechanics aplenty to moderate how faith works within the game. For some people, you see, faith is so strong that it manifests itself in physical effects - in game terms, these individuals have the Pure Faith Talent and can access specific faith powers. There's overlap with material in other supplements, particularly The Inquisitor's Handbook, and if you are already using those you'll have to decide if you want to mix and match them with these rules, which have been designed as a stand-alone system. If you want both, don't worry, they willl work together.
Next, Chapter 5: Reliquary provides loads of weapons, armour and gear for Ecclesiarcy members... after all, they do say that when you go to preach, take a big cudgel! It's not just weapons, though there are some quite nasty ones, this section also has devotional books, religious trinkets and much more. Or perhaps a character wants his armour decorated with religious symbols? Then there are strange servitors and somewhat more normal retainers, and services that you may wish to purchase - such as a baptism or a blessing for a weapon or person. The chapter ends with a discussion of relics and their uses.
Finally, Chapter 6: Ecclesiarcy Campaigns has a wealth of information and ideas about how to weave Ecclesiarcy themes through your campaign, as well as several senior Ecclesiarcy figures in the Calixis Sector to use as allies or enemies to your party. Learn how to make faith an integral part of your game and devise encounters and adventures with a religious theme. Despite the game's name, there's more to it than chasing down heretics and beating off aliens!
A thought-provoking book that really brings home how pervasive faith is throughout the Imperium, it makes for good reading to understand the setting further, even if you do not choose to use the rules herein, preferring to stick to 'straight' Acolytes and Inquisitors.