So this is the situation. I used Reverence (chose the result while praying) as my last spell on a Graveyard.

And if I want to chose 6 as my roll? I think that I should draw another spell right after using Reverence for Prophetess ability to always has a spell, and then take a spell for "rolling" 6 as an evil character on a Graveyard. Is that right? Thanks a lot.

That is correct.

You would draw another spell straight after casting reverence as per Prophetess ability, and then you would gain a spell as per the 6 result.

Such is the potency of Reverence.