This topic in fact comes quite close to one I had recently posted:
http://thetolkien.forum/showthread.php?19965-The-interventions-of-Eru
For the destruction of
Numenor is indeed one of these interventions, which I too have a hard time understanding.
Why does
Eru intervene and fight
evil in some spots, and let the world sort it out itself in others?
If Eru is Good, why not just destroy Evil, why not just splat
Melkor like a fly right when he start rebelling in the Music? The thread I posted pretty much asked the question whether Eru can be called "Good", for Evil also came from him, and Evil is part of the World, and Sadness adds to its Beauty. As there is nothing besides Eru in the beginning, everything, including Evil, must come forth from Him, so he can not only be Good.
In this case I think, we could say Eru intervened because he had too. The main reason was not that the
Numenoreans became Evil and threatened
Valinor, for as said, the
Valar should be able to easily crush this threat, and Eru doesn't strike down any other Evil
men (what about
Saruman, he was even wiser then the Numenoreans).
Instead, what forced Eru to intervene was the fact Men threatened to destroy his ordering of the world. Men were not supposed to become Immortal (apparently
Elves becoming Mortal was ok), Men weren't supposed to live in Amman.
What doesn't work here ofcours is
the simple fact, again, that the Valar could have crushed them themselves, and the fact some Mortals (
Bilbo,
Frodo and probably
Gimli and
Sam) did in fact turn up in Amman. Valar were obviously allowed to kill Men, for this could hardly have been avoided in the
War of Wrath.
Another intervention by Eru, resurrecting
Gandalf, perhaps could be because
Middle Earth wasn't supposed to be covered in Darkness untill the End of Times, but he let ending the Darkness of
Morgoth in the hands of the Valar too. Or maybe Gandalf would actually
die, being send to Middle-Earth in the body of a Man, and
Maia weren't supposed to die and leave
Arda. But then, what happened to Saruman, and
Sauron, and many others?
For myself, I still have a hard time with these parts of
Tolkien's works, possibly because I've never been religious. Instead, I like to think these part of the stories are part legend as told from father to son by Elves or Men, and not neccesarily complete facts. (Some characters in Tolkiens works in fact spread false wisdom, so not everything said is supposed to be true.)
I like to think that the Valar were actually the ones destroying Numenor (and resurrecting Gandalf), but because they are not Powers of Fear, they never claimed this as their own action, which led to Elves of Middle Earth to believe it was Eru himself.
Edit:
In short, in my vision
God/Eru can logically be one of several things:
1. Good, but not All-Powerfull, so he is not able to completely crush Evil. Would sometimes intervene, but can't fix everything.
2. Impartial, all the Universe comes from Him, and is part of Him. He would never intervene, for also Evil things are part of the "plan".
3. A kid with an antfarm. Sometimes changing/fixing something and watching what will happen next.