Well if we wanted direct translations, it would be "Faith is for the transient humans". For a surface reading, "Faith is for humans, who are transient" would be clearer, but the former captures the idea/vagueness better.
This page speculates that:
1) Humans are transient since they need the blessings of gods even continue to exist, eg: bountiful harvest, rain, etc, and
2) gods cannot give blessings if they don't receive faith (which was a the point in MoF, I think).
This page quotes the above page, and combines the above two ideas into:
o) Without faith, the transient humans would perish.
It then looks at this combined idea from the other side, obtaining:
o) Faith is for the transient humans.
(The second page also speculates that the title could be referring to Sanae herself, as she is herself a living god but is also a physical, and hence transient, human that serves the gods (Kanako and Suwako) as their priestess.)
And I'm not grabbing these just to set, like, 'the' interpretation, I'm just gathering different sources' ideas to add more ideas for the sake of interest and variety. Sharing interpretations is infinitely more fun than trying to or having one set in stone.
I suggest an alternate reading can also be obtained:
o) Faith is for the sake of transient humans.
Whether the 'for the sake of' implies: for the benefit of, or in the interest of, or in the favor of, or beneficial to, etc etc, is a personal reading.
In other words, because faith exists, gods can bless humans and they can continue to live, exist, and reap the joys and sorrows that come with them.
I think it fits well with her image, as the phrase 'as fleeting as the wind' is exactly the idea of transience. I'd imagine some people from various countries wouldn't be able to attest to the wind being described as fleeting though.