[Iarwain]

I must say that #2 was very clever, and now that I think of it, more probable, Manwe. If Gandalf was saying that he was a servant of the Flame Imperishable, than he would be "uncloaking" himself much more than if he were saying that he was the bearer of Narya.

The Flame would be Secret, you might recall because Morgoth had struck discord in the Music of the Ainur before Eru revealed to the Ainur Eä, and before that, Morgoth had longed for the Flame Imperishable, and had searched the void for the "secret fire". Also, Gandalf was sent by Manwe and Varda, Valar, and also direct servants of Eru. Therefore by serving them, he would also be serving Eru, creator of the Flame Imperishable.