Even if Sauron was lying to the Numenoreans, telling them that Eru was just a name, a ghost used by the Valar to keep them away from their land, he knew the truth! Melkor himself could not have done anything to Eru, being just an offspring of His thought; and Melkor understood this, turning all his hate against those that he could hurt (Valar, Maiar, Elves, Men and Dwarves).
Also, I think Tolkien made a good choice when he left out of his books explicit religion. Thus, his work has a more universal character which, otherwise, would have been restrained by the religious differences. Let's not forget that Tolkien, while a fervent catholic, always wanted to reveal the essence hiding behind the words. His masterpiece is, I think, the concept of Eru, which is an expression of God as He should've been in all religions. The name itself explains the character's nature: Eru ("The One", "He that is Alone") Iluvatar ("Father of All"). So: Beginning and End, Alpha and Omega.
That's all for now, but I'll be back with more ideas!
Evil may yet be good to have been ... and yet remain evil. (Silmarillion)