The Adeptas Sororitas are often referred to colloquially as "nuns with guns," and it makes sense because their faith in the Emperor is fundamental to their identity. That faith grants them abilities that others aren't able to tap into, and it's the special abilities that we get a look at in this preview of their upcoming 40k Codex.
From the article:
To say the Adepta Sororitas are a faithful bunch is selling them rather short, as their sheer fanatical devotion to the Emperor of Mankind is so strong that great miracles spontaneously occur whenever enough Sisters congregate. They must have been praying extra hard lately, as those miracles are becoming even more powerful.