... but being a devout Catholic, Tolkien would not create a godless world, or one with multiple gods (as many other fantasy writers have done). Hence, we have Eru/Illuvatar who is, I think, intended to represent God, at least in Tolkien's eyes.
The Ainur and the Valar are more like angels. This Istari must be some kind of lower cast angel (I like to think of Gandalf as an angel).
"May the Angels Guide"