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 the any inquisitive servant of the Imperium who survive their endeavors. It will inevitably happen, a sooner or a later as forbidden knowledge accumulates. If one is true to their their puritanical belief the step three should include a voluntary self-sacrifice in the name of the God-emperor. If not one has become a radical and an apostate in the eyes of the puritan factions.
The first step is a knowledge. An defender of the Imperium must understand basic traits of yhe 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. By allowing some aspect of the Chaos to survive or remain so that he can exploit or study it.
The third step is the line itself. When he becomes a radical and chooses to use the Chaos against the Chaos. When he employs agencies of the warp or when he asks the heretic for a help.
Source: Dan Abnett: Eisenhorn saga; Malleus, Chapter 20, edited for Ultima Tectum campaign.