I guess being a priest is not omnipotent. He doesn't have the skill to cure poison. His healing skill is only able to heal physical injuries and probably regenerate limbs?
So far as of chapter 44, his skills are:
Heal - healing injuries
Regen - replenish mp
Barrier - create barrier on your body
Skill rest - stop magic from being created
Magic circle - creates a stationary barrier
Magic circle search - it was not explained yet but I'm guessing it's a search magic?
(Did I miss any?)
If I had to guess, to cure poison he probably need specific skills like "cure" or "detox" or something.
I forgot his friend, Ren is not just poisoned. He's cursed, right? Yeah, probably need "dispel" or something for that.
They totally forgot about His title as a Priest? I thought he can heal everything..