Aegium

The Cult of Yan has the ability to certify Solfrey as Solfrey. It is understood that children of 2 Solfrey are generally always Solfrey. The children of a Solfrey and a non-Solfrey are typically not Solfrey. The children of 2 Commoners are typically Commoners. Commoners are carriers of demon souls, which is transferred to the children via the sperm and the egg.

The Yan detects the demon souls within the body of a babe, which delineates them as Commoner. If there is no demon soul, then the Yan performs a ritual of Aegium. The Yan then certifies the babe as Solfrey.

The Aegium birth rite is for every Solfrey. It's conducted by the parents and a third person, typically a Yan priest or a friend or grandparent holding the babe in their arms, with their arms crisscrossed with one another. Then they chant the "Behold, ye champion of Heaven, eternal ruler over the common.", and a symbolic magical spirit immunization spell.

The Yan priest/ess has the necromantic ability to inoculate the child against getting infested by a demon soul, and this is passively or magically done through this ritual. The ceremony may be performed by a parent of the child too, and the protection given is instinctual, but because they are not professionally trained they frequently (50% of the time) fail to actually protect the child. But it's better than nothing when you have to give birth without a Yan priest/ess around.