Is it heretical for a nun to give the homily?
The short answer
Not heretical, since heresy is the obstinate denial or doubt of a truth of faith. But it is wrong: the homily at Mass is reserved to a priest or deacon, never a layperson.
Heresy is the obstinate post-baptismal denial of a truth that must be believed with divine and Catholic faith, or obstinate doubt about it (CCC 2089), so this is not heresy.
Still, your mother is right that nuns and other laypeople should not preach the homily at Mass. Redemptionis Sacramentum (64) says the homily, part of the liturgy itself, should ordinarily be given by the priest celebrant; he may entrust it to a concelebrating priest or occasionally to a deacon, but never to a layperson. In particular cases, for a just cause, a bishop or priest present but unable to concelebrate may give it.
If she fulfilled her Sunday obligation at another Mass that day and left quietly without undue disruption, it was not wrong for her to register her displeasure at a significant liturgical abuse by leaving.
Continue exploring
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