"A letter from Pope Benedict V"
You need an exact row and column. One letter, two digits.
45-43-15/12-54/13-23-24-31-14-42-15-13/52-23-15-33/44-23-15-54/35-31-11-54/
45-43-15/12-54/23-45-32-11-33-43/52-23-15-33/44-23-15-54/43-31-11-54/
52-23-11-44/24-43/44-23-15/11-33-43-52-15-42/
Don't know the answer? What a shame!
Read again the letter, you might get a hint.
I challenge you to crack the code. Goodluck!