The fact that both "theological" and "message" (of the Author-God) are in quotes may be a clue about the context of the former here. It's provisional, derived from a sort of extended metaphor, where Barthes is using theology as a stand-in for whatever kind of strongly orthodox perspective (the orthodox perspective being one you touched upon in another question, wherein "The author still rules in manuals of literary history").
So there's an orthodoxy, and the metaphor Barthes is drawing is to theology, which, at least by most conventional definitions, pre-supposes the existence of the divine (or else it cannot proceed). Implicit in a conventional, or at least Judaeo-Christian conventional, conception of "divine" is that it by definition refers to the most fundamental level of existence, from which everything else flowed/flows. God created the world, it did not create him, etc.
To extend the metaphor, if the existence of the divine is fundamental to theology, then the existence of "the Author-God" (or Gods plural) is fundamental to the "manuals of literary history" that address their work. Bathes is contrasting this to a belief that
a text [...] is a space of many dimensions, in which are wedded and contested various kinds of writing, no one of which is original: the text is a tissue of citations, resulting from the thousand sources of culture.
He's not literally trying to claim that Shakespeare or Hemingway never existed, but that the fact of their existence is perhaps not all that significant to the existence of their work, which is never purely "original" and so on. Shakespeare, who's actual existence as a person who wrote all the things his name is attached to has been subject to question, is a good illustration of this; those arguments about his existence don't have much bearing on the interpretation or historical significance of the works he may or may not have written.
What Barthes is trying to dethrone is of course authorial intent, but also psychological interpretations that can be simply swapped in -- the existence of authorial unconscious intents should not be regarded as the ultimate truth about things any more than their conscious intents. Did Hemingway write the way he did about the things he did because of his childhood, repressed sexual desires, etc.? Perhaps (to keep extending the metaphor, a sort of "Mad God" scenario), but this is not much better or worse an explanation than that he was a genius craftsman (period, full stop).
Bathes's author is then a bit like the little man behind the curtain in the Wizard of Oz. The title of the book, "The Death of the Author" would seem to be a transparent reference to Nietzche's "God is Dead", which has a very different spin than the straightforward atheist premise that there is no God and never has been. Worth noting that in the same breath Nietzche writes "...and we have killed him" -- Barthes and his peer group would be mostly very aware of this passage. Further worth noting is that this is also a metaphor; Nietzsche is not waxing theological (although what the other half of his metaphor is may be more ambiguous) and that the death/killing is not necessarily to be considered a bad thing.
Likewise, the death of the Author-God is not necessarily a bad thing -- after all, Barthes is effectively exhorting us to help kill him. If the Author-God was a bit of a fiction to start with, then it is one which needs to die, not literally by taking to the streets and stoning writers but by detangling the fact of their existence from the existence of their works. This "killing" is like pulling back the curtain on the Wizard of Oz. Or Shakespeare, about which you can believe whatever you want: It will not make any difference to the text of The Tempest.