The Evidence is Overwhelming that Syntagma Was Used in Early Christianity as a Term Roughly Corresponding to 'Constitution'

Of course syntagma could be used in a different sense.  Later Christian writers like Photius certainly use it to denote any kind of writing.  But if we look at the manner in which the terminology is used in the earliest Christian writers it is clear that it means something like 'an arrangement to define a body of Christians.'  At least part of this may be attributable to the fact that prostagma is commonly used to mean 'decree':

Then shall He speak to them in His wrath, and vex them in His sore displeasure. Yet have I been set by Him a King on Zion His holy hill, declaring the decree of the Lord (τὸ πρόσταγμα κυρίου). [Psalm as cited in 1 Apology 40]

In the time of the lawless partisans of idolatry, wicked decrees (προσταγμάτων) were passed against the godly Christians in town and country, to force them to offer libations to vain idols; and accordingly the holy men, having been apprehended, were brought before the prefect of Rome, Rusticus by name. [Martyrdom of Justin 1]

We have already mentioned Justin's reference in his correspondence to the Emperor Antoninus Pius to his syntagma which will supposedly cure all the ills associated with contemporary Christianity.  Whether this reference was planted there by a later writer to justify the establishment of rules for the community, or whether Justin really did establish this new 'constitution' it is clear that the syntagma defined the shape of Christianity for the rest of history.

It is also interesting to note that much scholarly debate has been given to whether this syntagma is one and the same or different from the one Irenaeus says was directed against Marcion.  Eusebius seems certain that there was only one syntagma:

And the discourses of the man were thought so worthy of study even by the ancients, that Irenæus quotes his words: for instance, in the fourth book of his work Against Heresies, where he writes as follows: And Justin well says in his work against Marcion (τῷ πρὸς Μαρκίωνα συντάγματί), that he would not have believed the Lord himself if he had preached another God besides the Creator; and again in the fifth book of the same work he says: And Justin well said that before the coming of the Lord, Satan never dared to blaspheme God, because he did not yet know his condemnation. [Irenaeus Against Heresies 4.6.1; Eusebius Church History 4.18.9]

But Justin was especially prominent in those days. In the guise of a philosopher he preached the divine word, and contended for the faith in his writings. He wrote also a work against Marcion, in which he states that the latter was alive at the time he wrote. He speaks as follows: And there is a certain Marcion of Pontus, who is even now still teaching his followers to think that there is some other God greater than the Creator. And by the aid of the demons he has persuaded many of every race of men to utter blasphemy, and to deny that the maker of this universe is the father of Christ, and to confess that some other, greater than he, was the creator. And all who followed them are, as we have said, called Christians, just as the name of philosophy is given to philosophers, although they may have no doctrines in common. To this he adds: And we have also written a work against all the heresies that have existed, which we will give you if you wish to read it («ἔστιν δὲ ἡμῖν καὶ σύνταγμα κατὰ πασῶν τῶν γεγενημένων αἱρέσεων, ᾧ εἰ βούλεσθε ἐντυχεῖν, δώσομεν). [Eusebius Church History 4.11.10]

If Eusebius can be trusted about there being only one 'constitution' associated with Justin, it makes perfect sense why Celsus work is so hostile to the Marcionites.  This hostility was reflective of not only the age but also contemporary Imperial policy.

For those who want to see how the term syntagma is used to mean a document employed to govern a body of Christians, we furnish the following (partial) list of early examples:

keeping, then, to our aim, and selecting the Scriptures which bear on the usefulness of training for life, we must now compendiously describe what the man who is called a Christian ought to be during the whole of his life. We must accordingly begin with ourselves, and how we ought to regulate ourselves. We have therefore, preserving a due regard to the symmetry of this work, to say how each of us ought to conduct himself in respect to his body, or rather how to regulate the body itself. Στοχαζομένοις τοίνυν τῆς συμμετρίας τοῦ συντάγματος, ὁποῖόν τινα τῷ ἑαυτοῦ σώματι ἕκαστον ἡμῶν προσφέρεσθαι, μᾶλλον δὲ ὅπως αὐτὸ κατευθύνειν χρή, λεκτέον [Clement Paed 2.1]

But sparing prolixity in my treatise, I shall bring forward the following precepts of the Instructor, that you may guard against His threatenings. Ἀλλὰ γὰρ φειδόμενος τοῦ μήκους τοῦ συντάγματος τὰς ἑξῆς τοῦ παιδαγωγοῦ προσάγω σοι ἐντολάς, ἵνα δὴ φυλάξῃ τὰς ἀπειλάς. [ibid 3.8]

And faith is the possession not of the wise according to the world, but of those according to God; and it is taught without letters; and its handbook (σύγγραμμα), at once rude and divine, is called love--a spiritual ordinance book (σύνταγμα πνευματικόν). It is in your power to listen to divine wisdom, ay, and to frame your life in accordance with it. Nay, you are not prohibited from conducting affairs in the world decorously according to God. Let not him who sells or buys aught name two prices for what he buys or sells; but stating the net price, and studying to speak the truth, if he get not his price, he gets the truth, and is rich in the possession of rectitude. But, above all, let an oath on account of what is sold be far from you; and let swearing, too, on account of other things be banished. [ibid 3.11]

and the writings or volumes (τὰ συντάγματα) that consist of those holy letters and syllables, the same apostle consequently calls "inspired of God, being profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God may be perfect, thoroughly furnished to every good work." [Protrepticus 9.87.1]

Read the tractate entitled Against the Thirty-two Heresies (τὸ σύνταγμα κατὰ αἱρέσεων λβʹ) of Hippolytus, the pupil of Irenaeus. It begins with the Dositheans, and goes down to the heresies of Noetus and the Noetians, which he says were refuted by Irenaeus in his lectures, of which the present work is a synopsis. The style is clear, somewhat severe and free from redundancies, although it exhibits no tendency to atticism. Some of the statements are inaccurate, for instance, that the epistle to the Hebrews is not the work of the apostle Paul. [Photius Biblio. 121]

In the third epistle on baptism which this same Dionysius wrote to Philemon, the Roman presbyter, he relates the following: "But I examined the works and traditions of the heretics (τοῖς συντάγμασιν καὶ ταῖς παραδόσεσιν τῶν αἱρετικῶν ἐνέτυχον), defiling my mind for a little time with their abominable opinions, but receiving this benefit from them, that I refuted them by myself, and detested them all the more." [Eusebius Church History 7.7.1, Dionysius of Alexandria]

The occasion of these was Nepos, a bishop in Egypt, who taught that the promises to the holy men in the Divine Scriptures should be understood in a more Jewish manner, and that there would be a certain millennium of bodily luxury upon this earth. As he thought that he could establish his private opinion by the Revelation of John, he wrote a book on this subject, entitled Refutation of Allegorists. Dionysius opposes this in his books on the Promises. In the first he gives his own opinion of the dogma; and in the second he treats of the Revelation of John, and mentioning Nepos at the beginning, writes of him in this manner. But since they bring forward a syntagma of Nepos (ἐπεὶ δὲ σύνταγμά τι προκομίζουσιν Νέπωτος), on which they rely confidently, as if it proved beyond dispute that there will be a reign of Christ upon earth, I confess that in many other respects I approve and love Nepos, for his faith and industry and diligence in the Scriptures, and for his extensive psalmody, with which many of the brethren are still delighted; and I hold him in the more reverence because he has gone to rest before us. But the truth should be loved and honored most of all. And while we should praise and approve ungrudgingly what is said aright, we ought to examine and correct what does not seem to have been written soundly. [Eusebius Church History 7.24.1 - 4]

Agapius, Manichaean pamphlets - The utility of his impious and valueless syntagma (σύνταγμα) is solely to confuse and to the shame of those who are attached to the impious belief of the Manichaeans and to his own. [Photius Bibliotheca 179]

Some of the things about sects and schisms which I shall be telling the reader, I owe to my fondness for study. Certain things I learned from hearsay, though I happened on some with my own ears and eyes. I am confident that I can give an account, from accurate report, of the origins and teachings of some sects, and part of the what goes on among the others. Of these latter, I know one from the works of ancient authors (διὰ συνταγμάτων παλαιῶν συγγραφέων), another by listening to learned men who confirmed my notion precisely. [Epiphanius Panarion 1.1.2]

Their (= Carpocratian) syntagmas (τὰ αὐτῶν συντάγματα) are such that the intelligent reader will be astounded and shocked, and doubt that human beings can do such things—not only civilized people like ourselves, but even those who live with wild beasts and bestial, brutish men, and all but venture to behave like dogs and swine. For they say they absolutely must make every use of these things, or their souls may depart shy some work, and so be returned to bodies, to do all over again what they have not done. [ibid Pan. 1.27.5]

[Marcion] uses these two volumes (of the Bible) but has composed other syntagmas himself (ἄλλα δὲ συντάγματα ἀφ'ἑαυτοῦ συνέταξε) for the persons he has deceived. [Pan 1.42.9]

Elenchus 1 and 40. In agreement with the Epistle to the Ephesians, Marcion, you have also gathered these testimonies against yourself from the so-called Epistle to the Laodiceans. Thus, at the end of the syntagma (τῷ τέλει τοῦ συντάγματος), we may find what you have to say by reading it and, by finding what your teachings are, see through your heretical inventions, the three first principles with no first principles of their own which are different from each other. [Pan 1.42.11.2.182]

It would be interesting to see if we can find any examples of syntagma being used specifically as a term meaning 'constitution of a voluntary association.'  That might be a tall order to pick out, but it would settle this once and for all.