During her events of the study of the bible, you should have picked the option to question the bible and the religion. You need a certain amount on the INT stat for that.
And with that some of the following event would have either been a little different, or completely different.
Seeing you already have 85 of love, you basically lost this option. So you should first finish her romance with her full purity and marry her. To get the achievement.
Reload the game before proposing.
And then use the hypnosis skill to reduce her purity and corrupt her.