The Leopoldites actually got fucked over by the injections.

In short had they not taken the injections, they would fared a much better chance against Stig. I think it was not a failure on the Leopoldites faith but rather the fact that they took the injections that made their true Faith corrupted. Furthermore whilst Delirum would be terrible, it didnt really change much whether they were immune or not as shown with how the fight played out. Sure they would be easier to break now but considering Stig was massacring them with even needing the Warform shows that they didnt really need it. There biggest concern was rather whether their true faith was corrupted or not. Sure you could argue that the Leopoldites are morally bankrupt, but they certainly believe in their faith enough to be able to use it for True Faith in which whether you are personally a moral person never actually matter as along as you actually believed in your faith. The question is whether their Faith was corrupted by an external force in which from the scene with the Wyrm Corrupted Jesus yes, it was corrupted but not specifically because of their own beliefs or hypocrisy.