Silmarillion - Ainulindale

[Aldarion Elf-Friend]

Originally Posted by Macalaure

But the Ainur were the offspring of Eru's thought, so Melkor's vanity and lust for power had its source in Eru. This way, again, evil has its source in Eru.

This is another interesting depature from Tolkien's traditional Christianity. Eru says very straightforward that:

Originally Posted by J.R.R. Tolkien

...thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite.

This kind of moves toward the hyper-Calvanist position that minimizes the idea of free will, and stands in contrast to the more comfortable idea Eru simply takes what Melkor devised for his own glory, and uses it to make the whole that much more glorious.