According to the saint's revelation, the warring gods Ardia and Versa are waging a spiritual war over human souls. Mankind's real enemy are not the monsters, but instead the curse of voidtouch, which, if left untreated, will ultimately result in Ardia's destruction and Versa devouring the souls of humanity.
The transcript below is what Micah Sellic tells the party during the dinner aboard the Viator.
Spiritual battle over souls
"Everyone knows that this world is a battlefield of two warring gods: Father Ardia, our creator, and the evil goddess Versa, the mother of monsters. We see only a small fraction of this war spill into the physical world, in the form of monsters. But the real battle is fought on a spiritual battlefield, and it is fought over our souls."
Curse of voidtouch and frenzy
"We've come to believe that the monsters are the enemy we must fear. For a long time this made sense, for the monsters certainly want to kill us all. However, thanks to the revelation of our saint, we now know that what we really need to fear is the curse of voidtouch. For while the monsters pose a threat to our physical bodies, it is the curse of voidtouch that dooms the human soul."
"The evil goddess Versa, in Her infinite wickedness, seeks to wipe us out and devour our souls. To accomplish this, She uses her powers to continously reach out to human souls, causing them to become voidtouched. For the rest of their lives, the souls of the voidtouched are dragged towards Versa and upon the death of their host body, She will feast on those souls."
"In many cases, the soul is forcefully ripped from the body even before the body expires, and with no soul left to guide it, the body is forced to carry out Versa's order to destroy humanity. This is what we call frenzy."
Soul Rescue
"But our Father in heaven loves each and every one of His children, and will not let any human souls be lost to Versa. When the soul is threatened, Father Ardia uses His own divine power to pull the soul back to Himself, so that it may be reborn on Novrand. We call this process Redemptio Animae, or in your language, Soul Rescue."
"However, at this point the curse has been so deeply rooted, that Ardia is forced to spend more power to rescue the soul. Thus, every time Versa forces Ardia's hand in this manner, the power balance shifts in Versa's favor. Over time, this power balance will shift so completely in Versa's favor that there will be nothing left of Ardia to save us from our fate."
Soul Cleansing
"This situation seems completely hopeless, but there is a way out. Our very own Saint Lucia. Our little angel."
"With her miraculous blessing, she is able to channel and amplify Ardia's power to cleanse the human soul of the voidtouched curse. A soul cleansed this way can never become voidtouched again, and they are saved forevermore. We call this process Purificatio Animae, or Soul Cleansing."
"When the soul is cleansed by Saint Lucia, Versa loses the power She invested to curse that soul, and the power balance shifts instead in Ardia's favor. In essence, because Father Ardia cannot save a human soul in the physical world himself, He gave us Saint Lucia, who's able to do that in His stead."
Sellic later reveals that Soul Cleansing is a ritual that cannot be forced upon an individual. Rather, the subject must both seek it and accept it.
Final tally
"To give you a crude ballgame analogy: every time a voidtouched person is cured by Saint Lucia, we score 9 points for team humanity. And every time a voidtouched person dies, team humanity loses 1 point. This is why Lux Sanctus is willing to go to tremendous lengths to ensure that we can cure as many voidtouched as possible."