The New York Times tells me this morning that in the Georgia race for the U.S. Senate "the Warnock campaign wanted to make the race a contrast between a reverend and a running back."
Hmm. My copy of the Times stylebook (admittedly dating from 1999) and the current Associated Press Stylebook maintain the traditional distinction that reverend is to be used as an adjective (synonym of revered), never as a noun. And even as an adjective, there are restrictions: always "the Rev. Firstname Lastname" or "the Rev. Mr./Ms. Lastname," never "the Rev. Lastname" or "Rev. Lastname."
Among U.S. Christians, Episcopalians tend to be strictest about this convention, perhaps because they prize so many levels of reverence. Deacons and priests are "the Rev.," deans of cathedrals are "the Very Rev.," bishops are "the Right Reverend" ("the Rt. Rev."), and the presiding bishop is "the Most Rev." Bryan Garner says that use of the title without the article "has long been stigmatized as poor usage. And if the stigma is wearing off, it's doing so very gradually."*
But you know that U.S. Protestants, regardless of stylebooks, regularly use reverend as a title and speak of "Reverend Lastname" and describe that worthy as being "a reverend." (Reverend, incidentally, has been in use as a noun in English since the early part of the seventeenth century.)
Religion, I used to tell my editing students, is a thicket in which one quickly becomes entangled. All the Christian denominations have varying titles and practices, making it very easy for a writer to look like a fool. The problem, as Merriam-Webster's Dictionary of English Usage sums up, is that "there seems to be considerably greater acceptance of 'Reverend Moomaw' than most authorities recognize." And "this is really a matter of etiquette more than linguistic propriety, and the preference of the clergy involved should be taken into account if it can be determined."
So what are you, trying to sort out Reverend and the cluster of other current titles (Father, Mother, Brother, Pastor), going to do? You will first have to ascertain the conventions of the denomination you are writing about and then, if possible, the preference of the individual subject. Luck to you.
As for "a reverend," I'd steer clear of it as still too colloquial and allow it only in direct quotation.
* That citation is from Garner 4. My copy of Garner 5 has not yet arrived; when it does, I'll look to see if the entry has been revised.