McCulloch is an accomplished Historian and writer, but will naturally be biased against orthodox christianity (Catholicism) as he was raised in the Anglican (protestant) tradition and was almost ordained a priest in it. The reason he chose not to become ordained is due to his homosexuality, which is another personal circumstance which will cause him to reject orthodox christianity. He has been a member of the Lesbian & Gay Christian Movement since 1976.
He is no doubt a brilliant man, (see his achievements and posts he has held),but, due to aspects of his own background, I would suggest he is by no means an impartial commentator on Catholicism.
From his wiki page:
I was ordained Deacon. But, being a gay man, it was just impossible to proceed further, within the conditions of the Anglican set-up, because I was determined that I would make no bones about who I was; I was brought up to be truthful, and truth has always mattered to me. The Church couldn’t cope and so we parted company. It was a miserable experience
While I sympathise with the obvious disappointment and hurt evident in his words here, it seems he (like many) chose his sexuality over his Christianity and has since set about trying to change Christianity’s view of human sexuality.
He was wrong about “the Anglican set-up” incidentally - the CoE now seems on the verge of appointing openly gay bishops.