Crossing the line
“Everyone is damned. Mankind is damned. The whole human species. Chaos and death are the only real truths in reality. To believe otherwise is ignorance. And the Inquisition… so proud and dutiful and full of its own importance, so certain that it is fighting against Chaos… is the most ignorant thing of all. Your daily work brings you closer and closer to the warp, increases your understanding of orderless powers. Gradually, without noticing it, even the most puritanical and rod-stiff inquisitor becomes seduced.”
-Sadistic idolator and abomination Pontius Glaw-
There is a thin line between order and chaos, between right and wrong, between mankind and man-unkind. The line between puritanism and radicalism.
Crossing this line will happen to all inquisitive servants of the Imperium whom survive, sooner or later.
The first step is a knowledge. An defender of the Imperium must understand basic traits of chaos in order to fight it. In a few years he knows more about the warp than most untutored cultists.
The second step is a moment he breaks the rules and allows some aspect of Chaos to survive or remain so that he can study it and learn from it.
The third step is the line itself. When he becomes a radical. When he chooses to use Chaos against Chaos. When he employs agencies of the warp or when he asks the heretical for help.
Source: Dan Abnett: Eisenhorn Malleus Chapter 20, edited for Ultima Tectum campaign.