More from Sayers:
This tension between joy and the opposite of joy is, once again, something that is viewed with a certain distrust by an age committed to the pursuit of happiness. It can be readily pigeonholed. as lack of adjustment or, in severe cases, as a psychosis. In very severe cases it may indeed be a psychosis. But we must not disguise from ourselves that happiness is a gift of the heathen gods, whereas joy is a Christian duty. It was, I think, L.P. Jacks who pointed out that the word "happiness" does not occur in the Gospels; the world "joy", on the other hand, occurs frequently—and so does the name and image of Hell. The command is to rejoice: not to display a placid contentment or a stoic fortitude. "Call no man happy until he is dead", said the Greek philosopher; and happiness, whether applied to a man's fortunes or his disposition, is the assessment of something extended in time along his whole career. But joy (except for those saints who live continually in the presence of God) is of its nature brief and almost instantaneous: it is an apprehension of the eternal moment. And, as such, it is the great invading adversary that can break open the gates of Hell.
from "The Poetry of Search and the Poetry of Statement" (1963) as reprinted in A Matter of Eternity.