Board Thread:Warhammer 40k General Discussion/@comment-6134883-20130417202949/@comment-8257990-20130418200742

The Grey Knights, "greatest," of all who combat Chaos know that faith is possibly the very greatest tool against both heretics and the Daemons of Chaos. They know that most willingly turn to Chaos because they are the "best," thing to turn to (Or at least the corruption makes them feel this way) and believe that if your faith in the Emperor is so great then it helps massively to resist the temptation...

To be honest though the Grey Knight do look on the Sister as tools rather than people. There was one event where a squadron of Grey Knight terminators butchered hundreds of Sisters and coated themselves in the Sister's blood in order to repel corruption (even though they were nearly immune to this corruption and used the Sister's blood "just in case.")