How Powerful was Gandalf the Grey?

[Zigûr]

Originally Posted by Galadriel55

Alternatively, there is a possibility that can apply to either scenario: Gandalf was not altered internally, but rather was lent the power from the outside to carry out his task with a greater authority. Whether the power in question be Manwe or Eru

I would say Eru, because it was Eru who made the decision and had the power to send him back.

As R.R.J. has pointed out, in Letter 156, Professor Tolkien does indeed write that Gandalf was "enhanced in power", although the professor qualifies that this means "in sanctity": he was more holy as a result of his experience, perhaps more "pure". I think that might be the difference; as Gandalf the Grey, he was the representative of the Valar. As Gandalf the White, he was the representative of Eru Himself. This is an enhancement because it is an elevation of role and responsibility. It's also a message to Sauron, who since the end of the Second Age had convinced himself that Eru had given up on Arda and no longer cared what happened there.

I don't think Gandalf the White had more "power" in that he could cast "level 7 spells" instead of just "level 6 spells" or something. On the other hand, his greater "sanctity" would have made him more of an anathema to spiritually unnatural beings like the Ringwraiths.