Puctuation is a wonderful thing when used correctly.
Whosoever, monk or priest, sees this, may they take it. I have offered.
I've added parenthetical commas. They mean the same as this:
Whosoever (monk or priest) sees this, may they take it. I have offered.
Parenthetical commas whisper little interruptions that should leave behind a grammatically correct and meaningful sentence if completely removed.
's/he' means the same as the singular 'they' but 's/he' is far more difficult to pronounce. Odd to see in a quote.
"I have offered" sounds odd since it's typically a preposition but might simply be way to announce the offering. If we don't take it as a preposition, it's a full blown sentence on it's own.
"I've offered" is informal and seems out of place next to the rest of this formal speech.
With these changes "Whosoever" is perfectly correct.