Timothy is one of the three epistles known collectively as the pastorals (1 Timothy, 2 Timothy, and Titus). They were not included in Marcion's canon of ten epistles assembled c. 140 CE. Against Wallace, there is no certain quotation of these epistles before Irenaeus c. 170 CE.
Norman Perrin summarises four reasons that have lead critical scholarship to regard the pastorals as inauthentic (The New Testament: An Introduction, pp. 264-5):
Vocabulary. While statistics are not always as meaningful as they may seem, of 848 words (excluding proper names) found in the Pastorals, 306 are not in the remainder of the Pauline corpus, even including the deutero-Pauline 2 Thessalonians, Colossians, and Ephesians. Of these 306 words, 175 do not occur elsewhere in the New Testament, while 211 are part of the general vocabulary of Christian writers of the second century. Indeed, the vocabulary of the Pastorals is closer to that of popular Hellenistic philosophy than it is to the vocabulary of Paul or the deutero-Pauline letters. Furthermore, the Pastorals use Pauline words ina non-Pauline sense: dikaios in Paul means "righteous" and here means "upright"; pistis, "faith," has become "the body of Christian faith"; and so on.
Literary style. Paul writes a characteristically dynamic Greek, with dramatic arguments, emotional outbursts, and the introduction of real or imaginary opponents and partners in dialogue. The Pastorals are in a quiet meditative style, far more characteristic of Hebrews or 1 Peter, or even of literary Hellenistic Greek in general, than of the Corinthian correspondence or of Romans, to say nothing of Galatians.
The situation of the apostle implied in the letters. Paul's situation as envisaged in the Pastorals can in no way be fitted into any reconstruction of Paul's life and work as we know it from the other letters or can deduce it from the Acts of the Apostles. If Paul wrote these letters, then he must have been released from his first Roman imprisonment and have traveled in the West. But such meager tradition as we have seems to be more a deduction of what must have happened from his plans as detailed in Romans than a reflection of known historical reality.
The letters as reflecting the characteristics of emergent Catholocism. The arguments presented above are forceful, but a last consideration is overwhelming, namely that, together with 2 Peter, the Pastorals are of all the texts in the New Testament the most distinctive representatives of the emphases of emergent Catholocism. The apostle Paul could no more have written the Pastorals than the apostle Peter could have written 2 Peter.
The arguments that establish the inauthenticity of the pastoral epistles are expounded by Kummel in his Introduction to the New Testament, pp. 371-84. In addition to providing more detail to the arguments stated by Perrin, Kummel adds a few more considerations.
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