Joad looked at him with drooped eyes, and then he laughed. "Why, you're the preacher. You're the preacher. I jus' passed a recollection about you to a guy not an hour ago."
"I was a preacher," said the man seriously.
(John Steinbeck, The Grapes of Wrath)
There is the before preacher in the former sentence, but a in the latter. To what end are different articles used?
Joad uses the definite article because he is identifying the man not merely as a preacher, a member of the preaching profession, but as a specific preacher, the preacher about whom he possesses a memory which he recently passed on to another guy.
Casey uses the indefinite article because he is concerned not merely to acknowledge the identification but to distinguish his present from his former identity. He was a preacher, a member of the preaching profession, but he is so no longer: "Ain't got the call no more."
