Why did the prime minister refuse to discuss his faith?

Watch Boris Johnson's response after ITV News Political Editor Robert Peston asks him whether he is now a practising Roman Catholic

I am struggling to make sense of the prime minister’s answer to my question whether he is a practising Roman Catholic - which I asked in good faith and with good reason because he was recently married in Westminster Cathedral.

His answer was: “I don’t discuss these deep issues. Certainly not with you”.

He is aware that - for better or worse (worse for a long time) - this has been a pertinent question for chief and prime ministers since Henry Vlll.

More broadly, the professed faith or none of a leader matters to many voters. But it was the “certainly not with you” that took me aback.

There is nothing in my 35 years as a journalist to suggest that I would trivialise or denigrate religion, or any issue of conscience.

If someone had made that remark in that context when I started in journalism 35 years ago I might have replied “is that because I am Jewish?” or “is it because I went to a comprehensive and not Eton?”

But I assume none of that was on Boris Johnson's mind.

So if any of you - or even perhaps he - could explain, I would be grateful.