"Spiritual power." Rufus thought for a moment. The word spiritual brought to mind...
"Materia," he concluded. "Or rather, Mana, which is the measure of a person's ability to use certain types of materia. Materia is crystallized... Hm. Basically, it allows a person to use 'magic,' and the more Mana a person has, the more often and more powerful their materia use." He sounded like a textbook, he knew, except that Gast would be rolling in his grave to hear Rufus call materia 'magic.' Offering the good professor a mental nod, he continued. "Mana is often associated with spiritual strength, though the foremost authority on the matter insisted that the basic ability was rooted in biochemistry."
She was praying, in the fuzzy little recording Reeve showed Rufus: kneeling on an altar surrounded by water. Praying for them all, though no one knew it at the time.
"...Perhaps it was a little of both." Rufus shook away the wisps of the odd not-memory with a frown. "But the more often a person uses materia, the more Mana they develop— stronger with use, as with a muscle." Rufus himself had only been basically proficient with materia. "I admit, I never much bothered to build up my own strength." He grinned wickedly. "But then, I never needed materia to rule the world."