And that would be...?
The Fall of Numenor can be looked at in a variety of ways, so I'd be hesitant to reduce it to a clear and distinct religious message. But Tolkien obviously saw power as a big issue -- that it, by its very nature, is subject to abuse (thus Gandalf's famous speech about "At first I would use the ring for good, but...").
On a surface reading, we can say that Tolkien is proselytizing that humans shouldn't challenge the divine order/gods, that to disobey means death, so there are hints of Biblical wrath.
On a deeper level, it is an exploration of hubris and desiring power, in the context of seeking immortality as mortal beings. The Valar said, essentially, "you can't go West in a mortal form" -- which the Numenorians tried to do. Numenor was destroyed because they followed Sauron, who as a kind of recapitulation of Morgoth, sought to extend his power in the physical domain in a way that was out of harmony with the natural, which itself was the expression of the divine. In that sense, we can see echoes in a lot of apocalyptic fiction, where our technology gets ahead of ourselves by messing with the powers of nature, either atomically, environmentally, or genetically.
So I don't see Tolkien as presenting a heavy-handed religious message as much as he was exploring archetypal things about power, nature, etc. Because he was a devote Catholic doesn't make his messaging a kind of missionary apologism for Christianity, but his faith obviously influenced his work, if only by providing a basic ideological toolbox with certain archetypal themes, many of which have applicability across many cultures and ideologies (e.g. "The Fall" from a "Golden Age," which as
Richard Heinberg and others have explored, is one of the most universal myths). But that is true of his entire body of work, so I'm not sure why Numenor would be any more problematic than LoTR.
And of course "messaging" is only a problem if you don't like the message, or if it intrudes upon your suspension of belief and thus, enjoyment of the story. Fiction is filled with messaging of different kinds. I agree that if it gets too heavy-handed, with a sense that the creators are saying, "You should think this way, and if you aren't enjoying this and nodding your head, you're despicable," then it can be jarring, or at least annoying. I've never gotten that sense from Tolkien's work, unlike more overt religiously proselytizing fiction (e.g.
Left Behind, which I admittedly haven't read) or much of contemporary media with sociocultural themes.
