I'm placing my bets now:
1. Prostitute is a succubus
2. Alicia is the saint reincarnated
3. Prostitute made Alicia perverted so her powerful saintly magic would be weaker
4. The finale has something to do with 'love' and/or the consummation of it being more powerful than the succubus' 'temptation' or magic, so when Alicia and the monk finally enter a relationship and/or fuck, the day is saved and the succubi are beaten