[Blissey]

Tested this with Holy Spark with a staff on my off-hand. This is happening. Whenever you equip a staff on your off-hand, however, with Priest is main- or sub-class, your damage will instead decrease. If you don't have a staff equipped at all, God Rod's effect still applies. Wow, what a doozy.

Without Staff:
--- ROUND 0 ---
Doctor Mavis Haemin's turn.
Jammer's turn.
--- ROUND 1 ---
Doctor Mavis Haemin's turn.
Doctor Mavis Haemin's turn.
Jammer takes 90 Light damage.
Jammer takes 90 Lightning damage.
The battle has ended.
Gained 1 Mura.
You consume your emergency food supply.

With Staff:
--- ROUND 0 ---
Doctor Mavis Haemin's turn.
Doctor Mavis Haemin's turn.
Jammer takes 79 Light damage.
Jammer takes 79 Lightning damage.
The battle has ended.