a non-Catholic priest/minister do a short guest sermon, provided content was within Catholic teaching?
Episcopalian:
What about a non-Catholic providing diaconate or sub-diaconate services during the Mass?
What exactly is a “sub-diaconate”?
The Order of the Subdiaconate has been suppressed in the Latin Church and no longer exists in the Ordinary Form of the Mass. However, the Sub-Deacon is still liturgical role in the Extraordinary Form of the Roman Rite, the Anglican Use of the Roman Rite, and the order still exists in the Eastern Catholic Rites.
Sub-deacon was considered the first of the “Major Orders” - the major orders being: sub-deacon, deacon, and priest (the Bishop being considered a priest for this description).
It was also the highest “ordination” before receiving the Sacrament of Holy Orders for ordination to the Diaconate. The other orders were called the Minor Orders of Porter, Lector, Exorcist, and Acolyte.
A seminarian used to become a sub-deacon before of his second to last year in the seminary.
However, when Pope Paul VI suppressed the subdiaconate as an order in the Latin Church, he granted the Instituted Acolytes the ability to serve as a sub-deacon.
Therefore, today, when there are Solemn High Masses with with Deacons and Sub-Deacons in the Extraordinary Form, the role of the Sub-Deacon is either full filled by a priest, deacon, sub-deacon, or Instituted Acolyte.