How did Christianity cost Blair?
It didn't, but that is because Blair (quite rightly) recognised that it would be an error to bring his faith overtly into the job of being PM. While his faith, I'm sure, was a major motivational factor he never appeared to be making decisions overtly based on some aspect of christian doctrine.
I don't think May's religion cost her either. I doubt many people knew, nor cared, about her religion. And interesting that Vlad fails to mention Brown who is arguably more religious than either Blair or May - but again I don't think his religion was an issue.
Farron is different - because unlike the others, he overtly brought his religious beliefs into conflict with his job as a politician. That's what cost him, the conflict, not the belief of and in itself.
From what I've seen of Sunak, he seems much more like Blair, May, Brown etc - seems to be a practicing member of a religion but does not bring that overtly into his public role as PM. I think he will take a view on the PR value of his Hindu belief - on the one hand valuable in garnering support from a specific group in society balanced again the potential downside amongst certain racist elements for which this will cement a view of 'otherness'.