It was originally a colony ship, but then underwent a retrofit to to turn it into a warship. The Spirit of Fire is armed to the teeth. Not only is it heavily equipped for space combat, it is able to provide adequate support to ground engagements. To add on to that. The Spirit of Fire was equal to the size of a CCS-Class Battlecruiser. Meaning it it a HUGE ship.

Well, according to Halopedia, it was pretty heavily armed. It had a MAC cannon, which is basically the most powerful weaponry the UNSC had at the time.