As the red moon Dalamud was descending, Loisoix sent adventurers to pray at shrines to the Twelve, summoning them to aid Eorzea in hopes of preventing what came later to be known as the Calamity. Some subsequent conversations discussed the relationships between aether, gods, and the land itself:


Papalymo:
You have my thanks also. Truth be told, I had feared you would be reluctant to help. Summoning deities requires an ungodly amount of aether, after all. And having manifested, sustaining even a single deity's corporeal form costs the land dearly--as you've seen for yourself. Yet here we are, seeking to summon no fewer than twelve of them!

Channeling even a tiny portion of a deity's strength will tax you greatly. Fortunately, no more than that should be needed to stay the red moon's advance. At the very least, it should be sufficient to nullify Dalamud's power and ensure Eorzea's preservation.

For an added blessing, the process should see some aether returned to the land--aether that Dalamud had absorbed from the primals.
and a bit later:

Thancred:
These eyes will not be fooled--something weighs heavily on your mind. If I were to hazard a guess, the prospect of entreating the succor of the selfsame beings you once slew seems less than attractive? Heh, I'll not deny it's ironic.

As you well know, aether is the source of all life. Bereft of its blessing, Eorzea would be naught but a dried-up husk of a realm. By whatever name they're called, gods drink of the land's aether for their sustenance. The Twelve are no different. Summoning them would be tantamount to bleeding the life out of the land. Yet without their aid, Eorzea is most assuredly doomed. Dalamud will fall, and the land will die. Even should aether still flow, life may never again blossom here. We cannot allow that to come to pass.

Even as we move to stay Dalamud's descent, ever must we be mindful of the toll our actions take upon Eorzea's longevity. We spare the present at a cost to the future. It will be no easy thing, this balancing act. Yet we must see it through to the bitter end, no matter the hardship.