Sauron and Istari

[Sarumian]

Originally Posted by Calacirya

Sauron knows who the Valar are, and wouldn't be threatened by even a high elf, so it absolutely only makes sense if he assumed (even if it's not stated) that the Istari must be Maiar.

I think, Sauron would be threatened by anyone who was able to master The Ring, unless Sauron believed it was totally impossible. I don't think Gandalf and Galadriel were deceived by The Ring to the extant they were incapable of estimating their own potential.