3 Corinthians is a letter written in the name of Paul, supposedly in order to refute the heretical teachings of two individuals imposing themselves on the Christian community in Corinth. Named in the letter as Simon and Cleobius, these individuals espouse false teachings that are decidedly second century in nature. This offers good initial grounds to doubt the authenticity of the authorship of the text. Clearly, Paul would not have had the opportunity to address the concerns of Christians living during the century after his death. Yet although the content of the letter itself raises considerable doubt over its authenticity perhaps the most incontrovertible evidence centres on the vocabulary employed by its author. In the undisputed letters of Paul, Paul uses the word σὰρξ (flesh) in a very distinctive manner where it carries connotations of weakness and moral imperfection to the extent that flesh becomes intimately associated with sin and unrighteousness. In contrast, the author of 3 Corinthians uses the word far more casually and appears to be totally ignorant of Paul’s nuanced understanding of the word. Perhaps the most blatant example of this is where the author uses σὰρξ to describe the resurrection body – something Paul would never have done due to the connotations of weakness and moral imperfection that the term held for him. Indeed, this is in contradiction of Paul’s explicit denial that the resurrection body would be of flesh and blood in 1 Corinthians 15. It is my opinion that the anachronistic content of the letter and the differences in vocabulary constitute sufficient internal evidence to conclude that 3 Corinthians is a non-Pauline second century text.
Although the author may indeed have written 3 Corinthians in order to refute two historical individuals named Simon and Cleobius, it equally possible that they are simply literary creations onto whom the author projects the issues he wishes to discuss. In any case, the heretical teachings directly and indirectly addressed in the letter appear to be remarkably similar to mid second century Marcionite controversies. First, the letter goes out of its way to stress the common agreement between Paul and the other apostles, seemingly betraying familiarity with the Marcionite belief that Paul uniquely upheld the message of Jesus against the corruption of the other apostles. In fact, the author goes so far as to suggest that Paul was subordinate to the other apostles in the sense that Paul received the gospel message from those apostles who were in Christ before him. Of course, anyone familiar with Paul would recognise that this is the antithesis of Paul’s self-understanding – Paul claimed that he did not receive the gospel from any man, but through direct revelation from Christ.  In another apparent anti-Marcionite statement, the author explicitly identifies the creator God as both the God who raised Jesus and the God who sent the Prophets to Israel. This directly opposes the Marcionite belief that the Creator God of the Jews and the God of Jesus were two strictly separate Gods that were in hostile opposition to one another. Finally, the letter states that Jesus was born of Mary and of the seed of David in order to emphasise that Jesus came ‘in the flesh’. This too is in direct opposition to a Marcionite docetic Christology, which held that Jesus was not ‘born of a woman’ but descended from heaven with the appearance of a fully grown man during the reign of Tiberius. 
On the other hand, despite the author’s apparent preoccupation with disproving ideas that are widely known to have been held by Marcion it is worth noting that it could be premature to characterise the letter as peculiarly ‘anti-Marcionite’. After all, the author of the letter never explicitly mentions Marcion by name and it is possible that the author could have been writing in response to heresies held by another group of Christians independently of Marcion. For example, the heretical teachings addressed in 3 Corinthians also bear a remarkable similarity to the teaching of Saturninus recorded by Irenaeus.  Nevertheless, regardless of the exact identity of those perpetuating the heretical teachings, it is clear that the heresies incontrovertibly have their origin in second century Gnosticism. There is no example of any of these disputes in any first century work known to us. Perhaps the earliest precedent can be found in the letters of Ignatius, writing in the first decade of the second century, who wrote several letters defending orthodoxy against docetism.
Beyond the direct content of the letter, the vocabulary employed by the author also raises doubts about its authenticity. This is most apparent when comparing the use of the word σὰρξ (flesh) in 3 Corinthians and the undisputed letters of Paul.
An examination of Paul’s use of the word flesh demonstrates that it almost always holds a range of negative connotations. This is not to say that examples of a relatively neutral use cannot be found. Indeed, flesh is occasionally used by Paul to refer to the physical body or to physical kinship.  Perhaps the best examples of this is Romans 11:14, where he uses μου τὴν σάρκα (the flesh of me) to refer to his fellow Jews in terms of their common blood ancestry. Additionally, in 1 Corinthians 15:39, where he contrasts the different kinds of flesh given to man, animals, birds and fish, arguing that οὐ πᾶσα σὰρξ ἡ αὐτὴ σάρξ (not all flesh is the same). Yet overall, a neutral use of the term is infrequent in Paul. A far more common meaning, and still well within a context of physicality, is Paul’s use of the word to denote physical weakness and infirmity. Thus in Galatians 4:13, Paul refers to the state of being ill to δι’ ἀσθένειαν τῆς σαρκὸς, to being (in weakness of the flesh). In 2 Corinthians 7:5 he further emphasises the idea of the weakness of the flesh while describing his exhaustion, saying that his flesh has no rest because it faces conflicts on the outside and fears on the inside.
Elsewhere, Paul takes the connotation of weakness even further by associating flesh with moral inadequacy and imperfection.  For Paul, moral and physical weakness are intricately connected because the weakness of the flesh represents a root cause of the sinfulness of man. This means that Paul is able to use the flesh as a synonym for our sinful nature. Thus, we find that in Romans 8:3 the law is unable to fulfill its purpose because it was ἠσθένει διὰ τῆς σαρκός (weak through the flesh). And in Romans 3:20, the flesh is the very thing that cannot be justified through the works of the law. In a broader sense, one can view the flesh as the sphere in which sin is able to function. And by intimately connecting flesh with weakness and the operation of sin, Paul is able to contrast it with an alternative mode of being grounded in God. While the flesh is physical weakness and moral imperfection, the Spirit is its antithesis as a source of everlasting life. This gives rise to two modes of existence – one that orientated around the flesh and one that is orientated around the Spirit. Thus in Romans 8:6, we find that τὸ γὰρ φρόνημα τῆς σαρκὸς θάνατος, τὸ δὲ φρόνημα τοῦ πνεύματος ζωὴ καὶ εἰρήνη (the mind of the flesh is death but the mind of the spirit is life and peace). This dichotomy is also expressed In terms of the resurrection body. In 1 Corinthians 15:50 Paul argues that σὰρξ καὶ αἷμα (flesh and blood) cannot inherit the kingdom of God because of its intrinsic weakness and perishability. And in 1 Corinthians 15:44 we find that σπείρεται σῶμα ψυχικόν, ἐγείρεται σῶμα πνευματικόν (it is sown a natural body, it is raised a spiritual body). So although flesh and blood may not inherit the kingdom of God, a body that has been transformed by God and liberated from the weakness of the flesh into a body orientated towards the spirit certainly can.
Given that flesh clearly holds a rather particular meaning for Paul one would expect an authentic Pauline 3 Corinthians to conform to this definition to a reasonable degree. Yet its author repeatedly uses the word in a way totally foreign to Paul and which represents a blatant and irreconcilable contradiction of Paul’s understanding. While Paul believed in a physical resurrection body, he would never have used the word flesh, with all its negative connotations, to describe it. But in his eagerness to defend the physical nature of the resurrection body against heresy, the author of 3 Corinthians describes it using the word flesh as a synonym for literal physicality. In doing so, he has lost Paul’s close symbolic association of flesh with the realm of sin and his argument that the resurrection body cannot be of the flesh. This loss is demonstrated by the author’s statement that Jesus will ΕΛΕΥΘΕΡΩ ΣΗ ΠΑΣΑΝ ΣΑΡΚΑ ΔΙΑ ΤΗΣ ΙΔΙΑΣ ΣΑΡΚΟΣ ΚΑΙ ΕΙΝΑ ΕΚ ΝΕΚΡΩΝ ΗΜΑΣ ΕΓΕΙΡΗ ΣΑΡΚΕΙΚΟΥΣ ΩΣ ΕΑΥΤΟΝ ΤΥ ΠΟΝ Ε̣Ν̣ Τ̣Υ̣Π̣Ο̣Ν̣ ΕΔΕΙΞΕ (redeem all flesh by his flesh, and raise us up from the dead in the flesh like as he has shown to us in himself as an example). And again, in contradiction of Romans 3:20, the author argues that ΤΩ ΓΑΡ ΙΔΙΩ ΣΩΜΑΤΙ <ΧΡΣ> <ΙΗΣ> ΠΑΣΑΝ ΕΣΩΣΕ ΣΑΡΚΑ  (for by his own body Jesus Christ saved all flesh). These passages suggest that flesh can be saved in ignorance of Paul’s idea that it is inherently sinful and perishable. Significantly, this use of the word contradicts 1 Corinthians 15:50, where Paul argues that σὰρξ καὶ αἷμα (flesh and blood) cannot inherit the kingdom of God. In fact, the author of 3 Corinthians appears to be stating the total opposite to Paul – that flesh and blood can in fact inherit the kingdom of God. Thus in 3 Corinthians the resurrection body is not one that has been transformed from a body bound by its fleshly weakness into one clothed in the spirit. The author lacks any understanding of this transformation and Paul’s dichotomy between the flesh and the spirit is undermined.
In conclusion, it is clear that we have good grounds based on the internal evidence of 3 Corinthians to determine that it cannot be authentically Pauline and that it is likely to be a pseudepigraphal forgery written in the second century. This conclusion is further strengthened by the external witness of Tertullian and Eusebius, both of whom voice their doubts about the authenticity of the text.  It is nevertheless very interesting to compare these doubts with the writings of Aphrahat and his Syriac contemporaries, which show that the early fourth century Syrian Church considered 3 Corinthians to be an authentic Pauline canonical text. Indeed, Aphrahat quotes 3 Corinthians with the same formulae that he uses when quoting from other accepted Pauline texts in his Demonstrations. For example, in Demonstrations 6.12 a quotation of 3 Corinthians, “God divided of the Spirit of Christ and sent it into the Prophets”, is introduced by the phrase “The blessed Apostle also said.” In addition to this, Ephrem cites 3 Corinthians in his commentary on the Diatessaron, introducing the quotation with the phrase “And again the Apostle said this”.  Ironically, Ephrem even criticises the heretical Bardesanians for omitting 3 Corinthians from their canon! This confused external witness to the letter serves to demonstrate the important role that internal evidence can play in determining authenticity.
6. Greek quotations of 3 Corinthians are from http://www.textexcavation.com/documents/3corgrkeng.html