It is worth taking a second look at Serapion of Antioch:
It is probable that others have preserved other memorials of Serapion's literary industry, but there have reached us only those addressed to a certain Domninus, who, in the time of persecution, fell away from faith in Christ to the Jewish will-worship; and those addressed to Pontius and Caricus, ecclesiastical men, and other letters to different persons, and still another work composed by him on the so-called Gospel of Peter.
2. He wrote this last to refute the falsehoods which that Gospel contained, on account of some in the parish of Rhossus who had been led astray by it into heterodox notions. It may be well to give some brief extracts from his work, showing his opinion of the book. He writes as follows:
3. For we, brethren, receive both Peter and the other apostles as Christ; but we reject intelligently the writings falsely ascribed to them, knowing that such were not handed down to us.
4. When I visited you I supposed that all of you held the true faith, and as I had not read the Gospel which they put forward under the name of Peter, I said, If this is the only thing which occasions dispute among you, let it be read. But now having learned, from what has been told me, that their mind was involved in some heresy, I will hasten to come to you again. Therefore, brethren, expect me shortly.
5. But you will learn, brethren, from what has been written to you, that we perceived the nature of the heresy of Marcianus, and that, not understanding what he was saying, he contradicted himself.
6. For having obtained this Gospel from others who had studied it diligently, namely, from the successors of those who first used it, whom we call Docetæ (for most of their opinions are connected with the teaching of that school ) we have been able to read it through, and we find many things in accordance with the true doctrine of the Saviour, but some things added to that doctrine, which we have pointed out for you farther on. So much in regard to Serapion.
Carriker's conclusions:
The library at Caesarea thus contained Serapion's Letter to Domnus, Letter to Pontius and Caricus, and letters to some unidentified persons, as well as a treatise critical of the Gospel of Peter.
Indeed Eusebius's transcription of the letter appears to be faulty near the conclusion, Serapion's words being preserved in Greek as follows:
Ἡμεῖς δέ, ἀδελφοί, καταλαβόμενοι ὁποίας ἦν αἱρέσεως ὁ Μαρκιανός, ὃς καὶ ἑαυτῶι ἐναντιοῦτο, μὴ νοῶν ἃ ἐλάλει, ἃ μαθήσεσθε ἐξ ὧν ὑμῖν ἐγράφη, ἐδυνήθημεν γὰρ παρ' ἄλλων τῶν ἀσκησάντων αὐτὸ τοῦτο τὸ εὐαγγέλιον, τοῦτ' ἐστὶν παρὰ τῶν διαδόχων τῶν καταρξαμένων αὐτοῦ, οὓς Δοκητὰς καλοῦμεν τὰ γὰρ πλείονα φρονήματα ἐκείνων ἐστὶ τῆς διδασκαλίας, χρησάμενοι παρ' αὐτῶν διελθεῖν καὶ εὑρεῖν τὰ μὲν πλείονα τοῦ ὀρθοῦ λόγου τοῦ σωτῆρος, τινὰ δὲ προσδιεσταλμένα, ἃ καὶ ὑπετάξαμεν ὑμῖν
As Lampham notes "'meanness of spirit' (μικροψυχίαν) had been generated in the community and Serapion's first visit had failed to detect the underlying cause of the problem. On reflection, and with the benefit of expert advice, he recognized that he had been duped by certain of the brethren whose minds 'lurked in some hole of heresy' (αἱρέσει τινὶ ὁ νοῦς αὐτῶν ἐφώλευεν) influenced, as they were, by a certain Marcianus. His second visit would soon put matters right. The other matter of particular interest in Eusebius' account is that the Bishop of Antioch seems to have taken exception not to certain interpretative changes to the canonical text, but rather to a number of additions (προσδιεσταλμένα) to it." (Lampham, Peter the Myth and the Man p. 16)
Lampham also brushes aside those who claim that someone other than the leader of the Marcionite sect is meant here saying "it is often suggested that this Marcian was not the heretic of Pontus, but the leader of a Docetic sect at Rhossus. It should be noted, however, that twice previously (HE 4.22.5 and 5.) Eusebius has used the term 'Marcianists' in connection with Marcion, the founder of the well-known heretical sect." (ibid) Yet the truth is that this is not the only difficulty with this passage. It would seem to have been very badly translated from Syriac into Greek and so Schaff acknowledges great difficulties with his translation saying that "the interpretation of these last two clauses is beset with difficulty. The Greek reads τουτέστι παρὰ τῶν διαδόχων τῶν καταρξαμένων αὐτοῦ, οὓς Δοκητὰς καλοῦμεν, (τὰ γὰρ φρονήματα τὰ πλείονα ἐκείνων ἐστὶ τῆς διδασκαλίας), κ.τ.λ. The words τῶν καταρξαμένων αὐτοῦ are usually translated “who preceded him,” or “who led the way before him”; but the phrase hardly seems to admit of this interpretation, and moreover the αὐτοῦ seems to refer not to Marcianus, whose name occurs some lines back, but to the gospel which has just been mentioned. There is a difficulty also in regard to the reference of the ἐκείνων, which is commonly connected with the words τῆς διδασκαλίας, but which seems to belong rather with the φρονήματα and to refer to the διαδοχῶν τῶν καταρξαμένων. It thus seems necessary to define the τῆς διδασκαλίας more closely, and we therefore venture, with Closs, to insert the words “of that school,” referring to the Docetæ just mentioned."