With a cover featuring a flaming sword breaking through a pane of glass, "On Fire!" was Petra's heaviest album yet, the culmination of the transition begun with Back to the Street. A fully hard rock album, bordering in places on heavy metal, "On Fire!" made the most of songs such as "All Fired Up", "Hit You Where You Live" and "Mine Field" – all of which continued the spiritual warfare theme. "CCM Magazine", in praising the album, called "On Fire!" "the crunchiest Petra sound yet" and praised the first two tracks as "the most exciting eight minutes of rock Petra has ever put to tape." Once again, the band scored hits with its initial singles, as "First Love", the radio ballad, and "Mine Field", the rocker, hit No. 1 on three of Music Line Magazine's four Christian radio charts. "First Love" finished 1988 as the year's 10th-most-played radio single.