There is an interesting paragraph on page 363 (opening paragraph about the agents of the imperium). It mentions that the Imperium is a behemoth of human achievement and that, due to it`s sheer size, the followers of chaos and xenos are "bordering extinction" in comparison.
I thought about how different the Imperium looks outside looking in. When you read the standard warhammer 40K fare, the opposite seems true, humanity is beset from all sides and is on the verge of extinction. Xenos races on all sides attack human planets and Chaos seems to be everywhere.
However, from the point of view of a chaos follower, things might seem very different. Full blown chaos cultists probably make up only a tiny fraction of the total population. Summoning demons is very difficult to do and dimensional instability makes them disappear quickly. I have read in many short stories that, as scary as Black crusades are, they are usually beaten back and really do not any real long lasting damage to the Imperium as a whole (losing a few dozen planets out of a million worlds). Many Chaos marines have lamented that they have been beaten back to the eye of terror and that the attacks that they mount are rather ineffectual in the grand scheme of things. If one is a chaos cultist, he is going to also feel very weak compared to an imperial guardsman, or adeptus arbite enforcer, much less a Space Marine or Inquisitor.
As for Xenos, besides orks and Tyrannids, most xenos races are probably nearly extinct compared to humanity. The Eldar are a dwindling race, the dark eldar only have one shadowy realm homeworld, the tau have barely over 100 worlds to call their own. Most of the other xenos races are on an even smaller scale and get swallowed up with imperial expansion rather quickly. Even when looking at the orks, as a whole there are many of them, but they are extremely disorganized and fight among themselves much of the time. The tyrannids have more purpose, but many hive fleets have been defeated due to the daring of Space Marine Chapters (notably the Ultramarines who have been responsible for many Tyrannid defeats).
So, I guess the point of this thread is... is the Imperium outnumbered and about to collapse due to the enemies trying to overwhelm it, or are the enemies of the Imperium fighing a losing battle against the crushing weight of the Imperium? Who has the advantage?