Spirit's Bane — Julie C. Gilbert

The Kindred Spirit Bond makes them powerful ... Lucifer has noticed. As with all young angels on the Guardian path, Allister and Mina undertake a wide variety of missions to discover their Gifts. Meanwhile, Satan and his top lieutenants have a new secret weapon to test, so they set a trap for the pair. Now, the Guardian Angel candidates need protection. If Spirit's Bane successfully destroys the angels of light, the last line of defense between humans and demons will disappear.***This is Book 1 in the Guardian Angel Files series. Book 2 is Kindred Spirits. Both are available as audiobooks narrated by Reuben Corbett. It's Christian fantasy written in a style reminiscent of C.S. Lewis's Screwtape Letters.