You can't really classify Mavis--she's less a gospel singer or soul singer and more a piece of American history in musical form, and she uses her voice to channel her unique experience and her special spirit.
On this album she does sing quite a few gospel songs, and you can really hear how vital her faith is to her life. But her band is a rocking, rootsty, bluesy trio, her backup singers range from indie rockers (the Neko Case affiliated duo of Nora O'Connor and Kelly Hogan) to 70's-power ballad veterans (Donny Gerrard sang lead on a Top Ten hit for Skylark that Tupac sampled), and with Jeff Tweedy picking decidedly non-gospel songs by Fogerty, Randy Newman, Little Milton and Allen Touissant plus writing two new ones for her to sing, the record easily transcends the "gospel" tag. The result is a rollicking set of songs with the common denominator being Mavis' force of nature voice soaring, celebrating, whispering and goading you into smiling and nodding and singing along.
While not every song totally works ('Creep Along Moses' is a little too stuttery and off-the-wall for me), the halfway track, "I Belong To the Band" epitomizes the album: Mavis singing an old-timey standard with all her heart and soul and creating a joyful, jumping house party singalong with a fat bassline and a killer guitar solo. And even an atheist can love that...