"Sherlock," John says abruptly, the thought dawning on him for the first time yet suddenly becoming very obviously true, "Do you actually know the story of the Nativity?"
There is a very long silence. John is just about to laugh with amazement when Sherlock harrumphs.
"Of course I do, John, don't be absurd. What do you take me for?"
John wisely chooses to ignore that question and instead prompts him to continue. "Go on, then."
There is another prolonged pause. "...It involves shepherds."
"This is unbelievable!" An incredulous laugh bursts from John's mouth, Sherlock looking increasingly surly.
"Will it help me catch criminals? No. It's irrelevant. I probably deleted it years ago."
"That's what you said about astronomy." John points out reasonably as he goes to pour a cup of tea from the freshly-boiled kettle. The grumpy silence indicates that Sherlock seems to have no answer for him, so his flatmate takes advantage of this and proceeds to tell him the Nativity story – punctuated, of course, by Sherlock's many scathing criticisms and scoffs of disbelief.
–
"...So they travelled to Nazareth and made it their home. The end."
Sherlock sits in thoughtful silence, steepling his fingertips beneath his chin and staring into space, but just as John is starting to think he's made an impression-
"Teenager has an affair, tries to cover it up by saying she was impregnated by God, nine months later delusional old men follow a particularly bright star to harass the new mother and a cult leader is born."
"It's an iconic story!" John protests, giggling a little despite himself. "You can't just-"
"I can and I will." Sherlock says decisively, dismissing the matter with an airy wave of the hand. "And anyway, I was right – that farce of a tale couldn't help me with a case if my client were the baby Jesus himself."
Sherlock and religion don't work well together, at a guess.