If the Grey Order needs you to assassinate an Empress, you don't ask them for permission before assassinating the Empress, as that denies them the plausible deniability required to assassinate an Empress:
It's not impossible. The Grey Order operates that way in some circumstances, where forgiveness after the fact is sometimes granted when permission could never have been given if it had been sought beforehand.
And stealing Cult Secrets is on that same level of severity:
If your main priority is continuing the research, there's no benefit to telling Ranald. Either He's against it and bad things happen, or the consequences for what happens if it gets found out just got a whole lot worse for everybody because now Ranald is implicated.
So why
wouldn't the same principles apply here? Why is it "very unlikely" that Ranald would want deniability, if we started poking at the secrets of other gods?
Especially because, again,
Ranald was paying enough attention to us while we were chatting with Ljiljana to instantly hear her message and give us a response.
Mathilde spent several weeks preparing to run experiments with AV, the Coin, and magic signature crystals.
There's no way Ranald wouldn't at some point notice what we were up to, unless He figured out what we were doing and deliberately buried His head in the sand for the sake of plausible deniability in case the Cults see what we're up to.
He should have known, unless He didn't
want to know.