I often use it to mean "that which you do when you don't have instructions/good reasons to do something else." e.g. I tell my patients to sit up and move around; to lay in bed only when they need to rest and not as their default position.
In this case I get the impression that the divinity degree was not chosen for any specific reason or interest but was what "everyone" studied unless they had a specific interest in another area. I think this was the case historically. In what year is your book set?