For most wizards, faith isn't that important. The gods are up there doing god things, and wizards are down here doing wizard things, and as long as the wizard doesn't mess with the bad gods, then nothing more needs to be said. That's more or less the public stance of the Colleges in general (druids and the Bright's weird mystery cult thing notwithstanding).
But Mathilde has rejected that orthodoxy, and has instead cultivated a very personal relationship with one god in particular, and it has been a massive advantage to her as a wizard. Asarnil even said as much after the Kul encampment thing. I don't think it's wrong, as Eike's Master, to share that part of ourselves and highlight how a relationship with the divine can be of benefit—because it very much can, if the right gods take an interest.
In fact, ten year old Mathilde's reasoning for worshipping Ranald was:
"I can get a God to help me out just by doing the things I'm going to be doing anyway?"
Sharing that wisdom with our Apprentice is perfectly natural, and, in my opinion, incredibly interesting and I would like to read it. And I don't think religion is as private or as sensitive in the Warhammer world as it is in ours—it's just that we have a religious commandment to keep our faith hidden, and we mostly interact with people who don't have a faith, or it's such an integral part of their character that it doesn't need to be discussed.