Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Second Ecumenical Council:  The First Council of Constantinople.: Warning to the Reader Touching Canon VII.

Warning to the Reader Touching Canon VII.

(Beveridge, Synodicon, Tom. II., in loc.)

This canon, I confess, is contained in all the editions of the Commentaries of Balsamon and Zonaras.  It is cited also by Photius in Nomocanon, Tit. xii. ch. xiv., besides it is extant in a contracted form in the Epitome of Alexius Aristenus.  But it is wanting in all the Latin versions of the Canons, in the ancient translations of Dionys. Exig., Isidore Mercator, etc.; also in the Epitome of Sym. Logothet., and the Arabic paraphrase of Josephus Ægyp., and what is particularly to be observed, in the collection and nomocanon of John of Antioch; and p. 185 this not through want of attention on his part, as is clear from this namely, that in the order of the Canons as given by him he attributes six Canons only to this second General Council, saying “…of the Fathers who assembled at Constantinople, by whom six Canons were set forth,” so that it is clear the present was not reckoned among the canons of this council in those days.  Nay, the whole composition of this canon clearly indicates that it is to be ascribed, neither to this present council, nor to any other (unless perhaps to that of Trullo, of which we shall speak afterwards).  For nothing is appointed in it, nothing confirmed, but a certain ancient custom of receiving converted heretics, is here merely recited.

(Hefele, History of the Councils, Vol. II., p. 368.)

As we possess a letter from the Church at Constantinople in the middle of the fifth century to Bishop Martyrius of Antioch, in which the same subject is referred to in a precisely similar way, Beveridge is probably right in conjecturing that the canon was only an extract from this letter to Martyrius; therefore in no way a decree of the second General Council, nor even of the Synod of 382, but at least eighty years later than the latter.  This canon, with an addition, was afterwards adopted by the Quinisext Synod as its ninety-fifth, without, however, giving its origin.