Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The First Ecumenical Council:  The First Council of Nice.: Canon III

Canon III.

The great Synod has stringently forbidden any bishop, presbyter, deacon, or any one of the clergy whatever, to have a subintroducta dwelling with him, except only a mother, or sister, or aunt, or such persons only as are beyond all suspicion.

Notes.

Ancient Epitome of Canon III.

No one shall have a woman in his house except his mother, and sister, and persons altogether beyond suspicion.

Justellus.

Who these mulieres subintroductæ were does not sufficiently appear…but they were neither wives nor concubines, but women of some third kind, which the clergy kept with them, not for the sake of offspring or lust, but from the desire, or certainly under the pretence, of piety.

Johnson.

For want of a proper English word to render it by, I translate “to retain any woman in their houses under pretence of her being a disciple to them.”

Van Espen.

Translates:  And his sisters and aunts cannot remain unless they be free from all suspicion.

Fuchs in his Bibliothek der kirchenver sammlungen confesses that this canon shews that the practice of clerical celibacy had already spread widely.  In connexion with this whole subject of the subintroductæ the text of St. Paul should be carefully considered.  1 Cor. ix. 5.

Hefele.

It is very certain that the canon of Nice forbids such spiritual unions, but the context shows moreover that the Fathers had not these particular cases in view alone; and the expression συνείσακτος should be understood of every woman who is introduced (συνείσακτος) into the house of a clergyman for the purpose of living there.  If by the word συνείσακτος was only intended the wife in this spiritual marriage, the Council would not have said, any συνείσακτος, except his mother, etc.; for neither his mother nor his sister could have formed this spiritual union with the cleric.  The injunction, then, does net merely forbid the συνείσακτος in the specific sense, but orders that “no woman must live in the house of a cleric, unless she be his mother,” etc.

This canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars I., Distinc. XXXII., C. xvj.