Story Search Entry 1 · 8:50am Oct 28th, 2013
The story I was trying to find was about Celestia and Luna. The details I remembered were that:
it took place before they were rulers of Equestria;
they might be changelings in disguise (Note: they were);
if they were changelings, they were probably sent on a mission to Equestria (Note: they were);
they either claim to be goddesses or get asked if they are goddesses and say "yes" (Note: it was the latter);
there may be a reference to fakes having previously shown up (Note: there was);
the original date of publication was prior to 2013 (Note: I was wrong; it was 2013).
The mystery was solved when one notMurphy said that Zephyrus Scary had made a blog post about a similar story. He was wrong, but Zephyrus Scary was nevertheless able to identify the story as The Alicorn Delusion.
(Note: This all happened many moons ago, but I'm putting in in my otherwise-unused blog for posterity to free up the space it was occupying in my bio until now.)