Is Eru God?

[Aiwendil]

Davem wrote:

The true source of creation is Eru, who is sole possessor of the Secret Fire.

True, but consider Eru's words to the Ainur:

Of the theme that I have declared to you, I will now that ye make in harmony together a Great Music. And since I have kindled you with the Flame Imperishable, ye shall show forth your powers in adorning this theme, each with his own thoughts and devices, if he will.

So because they have been 'kindled' with the Flame Imperishable, they do have a degree of creative power. The theme is Iluvatar's but the Ainur 'adorn' it with their 'own thoughts and devices'. The role of the Valar is not, I think, merely to carry out the designs of Eru like 'living tools'; they embellish that design with their own free will - even if their creative powers are ultimately derivative of Eru's.