Agents of the Imperium
"Heresy is like a tree, its roots lie in the darkness whilst its leaves wave in the sun and to those who suspect nought, it has an attractive and pleasing appearance. Truly, you can prune away its branches, or even cut the tree to the ground, but it will grow up again ever the stronger and ever more comely. Yet all awhile the root grows thick and black, gnawing at the bitter soil, drawing its nourishment from the darkness, and growing even greater and more deeply entrenched.
Such is the nature of heresy, and this is why it is so hard to destroy, for it must be eradicated leaf, branch, trunk and root. It must be exorcised utterly or it will return all the stronger, time and time again, until it is too great to destroy. Then we are doomed."
Galan Noirgrim, Master of the Ordo Malleus