Ante-Nicene Fathers, Vol VIII:
The Decretals.: Part II

II.

(Of those who have intercourse with excommunicated persons, or with unbelievers.)

Those, too, who are excommunicated by the priests, let no one receive previous to the just examination of both sides; nor let him have any intercourse with such in speech, or in eating or drinking, or in the salutation with the kiss, nor let him greet such; because, whosoever wittingly holds intercourse with the excommunicated in these or other prohibited matters, will subject himself, according to the ordinance of the apostles, 2735 to like excommunication.  From these, therefore, let clergy and laity keep themselves if they would not have the same penalty to endure.  Also do not join the unbelievers, neither have any fellowship with them.  They who do such things, indeed, are judged not as believers, but as unbelievers.  Whence the apostle says:  “What part hath he that believeth with an infidel? or what fellowship hath righteousness with unrighteousness?” 2736

Footnotes

The reference is to the 11th and 12th of the canons of the apostles.  [Vol. vii. p. 501, this series.]

2 Cor. 6:14, 15.