Aziraphale winces at the outburst, mostly because he'd prefer to not cause a scene at the library. It's a relief when Sherlock storms out, and after a moment's hesitation, he decides to follow.

"God didn't want to destroy anything," he calls out to the man's retreating back, once he's standing on the steps. "Her plan's ineffable, you know!"

He sighs and leaves it at that. The poor detective clearly needs to figure out what is real on his own, and an angel's presence likely doesn't help matters. Maybe the next time they cross paths, they can have a polite conversation.