Baptism Sacrament

The sole miracle that members of the Holy Church are permitted to acquire.
It is a simple ritual, which sublimates lost souls and sends them to where they belong according to the teachings of the Lord.
Every school of magecraft has their own “magical theory,” a foundation carved into the world itself which they use to hijack the natural order (rules and systems) and activate spells.
The spells enacted by schools with little faith and small foundations will be weak, and it's not uncommon for them to stop functioning completely in foreign countries.
The magical theory with the largest, most widespread “foundation” is the Church's teachings of the Lord, the logos.
Though its physical interference power is feeble, its effect on spiritual bodies is tremendous.
As for why their teachings have become so widespread, it's probably because this “miracle that appeals to the soul” soothes the hearts of the masses.

Fate/side material: Fate Encyclopedia