There really is a paucity of info on Holy, grepping the main text all we can say is:
white1:
- Holy is a last resort for defending the Planet.
- Holy must be activated using the White Materia and by praying to the Planet.
- Holy destroys all that the Planet considers evil.
- Holy glows when activated.
(The Ancients, via Bugenhagen)
fship_25:
- Sephiroth must be defeated in order to release Holy (Cloud)
lastmap:
- One way or another, Holy has been activated, perhaps by Aerith (Cloud), perhaps by collective prayer (Barret).
las4_0
- It's up to the Planet to decide what to do with Holy (Cloud)
Ending:
- The Planet unleashes Holy
- If they don't get out of the Crater they're toast (Red)
- The Highwind is damaged by Holy as it tries to escape
- Midgar is heavily damaged if not destroyed by Meteor
- Holy arrives too late, Meteor is too close (Red)
- Holy is backfiring, putting the entire planet at risk (Red)
- Lifestream comes to save the day and defeats Meteor *with* Holy
- Fade to white
- Fast forward and we can see Midgar was never rebuilt
Conclusions:
It's hard to tell from the ending, but I get the impression that the Highwind "surfs" on top of Holy and avoids taking the full brunt. Red at least seems to think that being in the crater when Holy goes off means death.
Holy should have been enough, but it came too late. The video doesn't explain why Holy being too late matters, or why it would backfire and threaten the Planet, but I suppose we have to trust what Red says. The implication from Red is that Meteor was only a threat to Midgar until Holy showed up.
I can't wrap my head around this last part to be honest, how exactly is Holy backfiring? Why's the timing so important? So many questions...