We find these seven mentioned not only by Eusebius ("Hist. eccl.", III, xxxvi) but also by St. Jerome (De viris illust., c. xvi). Of later collections of Ignatian letters which have been preserved, the oldest is known as the "long recension". This collection, the author of which is unknown, dates from the latter part of the fourth century. It contains the seven genuine and six spurious letters, but even the genuine epistles were greatly interpolated to lend weight to the personal views of its author. For this reason they are incapable of bearing witness to the original form. The spurious letters in this recension are those that purport to be from Ignatius
It seems like the only things we can be sure of about the original seven epistles are their titles.
-ksen
Here is the next paragraph from the Catholic Encyclopedia, and the obvious source of the obsession of Calvinists and others to try to impugn the authenticity of the Ignatian letters:
Contents of the letters
It is scarcely possible to exaggerate the importance of the testimony which the Ignatian letters offer to the dogmatic character of Apostolic Christianity. The martyred Bishop of Antioch constitutes a most important link between the Apostles and the Fathers of the early Church. Receiving from the Apostles themselves, whose auditor he was, not only the substance of revelation, but also their own inspired interpretation of it; dwelling, as it were, at the very fountain-head of Gospel truth, his testimony must necessarily carry with it the greatest weight and demand the most serious consideration. Cardinal Newman did not exaggerate the matter when he said ("The Theology of the Seven Epistles of St. Ignatius", in "Historical Sketches", I, London, 1890) that "the whole system of Catholic doctrine may be discovered, at least in outline, not to say in parts filled up, in the course of his seven epistles". Among the many Catholic doctrines to be found in the letters are the following: the Church was Divinely established as a visible society, the salvation of souls is its end, and those who separate themselves from it cut themselves off from God (Philad., c. iii); the hierarchy of the Church was instituted by Christ (lntrod. to Philad.; Ephes., c. vi); the threefold character of the hierarchy (Magn., c. vi); the order of the episcopacy superior by Divine authority to that of the priesthood (Magn., c. vi, c. xiii; Smyrn., c. viii;. Trall., .c. iii);the unity of the Church (Trall., c. vi;Philad., c. iii; Magn., c. xiii);the holiness of the Church (Smyrn., Ephes., Magn., Trall., and Rom.); the catholicity of the Church (Smyrn., c. viii); the infallibility of the Church (Philad., c. iii; Ephes., cc. xvi, xvii); the doctrine of the Eucharist (Smyrn., c. viii), which word we find for the first time applied to the Blessed Sacrament, just as in Smyrn., viii, we meet for the first time the phrase "Catholic Church", used to designate all Christians; the Incarnation (Ephes., c. xviii); the supernatural virtue of virginity, already much esteemed and made the subject of a vow (Polyc., c. v); the religious character of matrimony (Polyc., c. v); the value of united prayer (Ephes., c. xiii); the primacy of the See of Rome (Rom., introd.). He, moreover, denounces in principle the Protestant doctrine of private judgment in matters' of religion (Philad. c. iii), The heresy against which he chiefly inveighs is Docetism. Neither do the Judaizing heresies escape his vigorous condemnation.
Here then we see why you must believe, erroneously, that the quote I posted is a forgery. For to believe the Truth is to become Catholic.
Of later collections of Ignatian letters which have been preserved, the oldest is known as the "long recension". This collection, the author of which is unknown, dates from the latter part of the fourth century. It contains the seven genuine and six spurious letters, but even the genuine epistles were greatly interpolated to lend weight to the personal views of its author. For this reason they [i.e., the "long recension" itself, not the originals] are incapable of bearing witness to the original form. The spurious letters in this recension are those that purport to be from Ignatius
St. Polycarp Martyr (A.D. 69-155).
Our chief sources of information concerning St. Polycarp are: (1) the Epistles of St. Ignatius; (2) St. Polycarp's own Epistle to the Philippians; (3) sundry passages in St. Irenaeus; (4) the Letter of the Smyrnaeans recounting the martyrdom of St. Polycarp.
The Epistles of St. Ignatius
Four out of the seven genuine epistles of St. Ignatius were written from Smyrna. In two of these -Magnesians and Ephesians- he speaks of Polycarp. The seventh Epistle was addressed to Polycarp. It contains little or nothing of historical interest in connexion with St. Polycarp. In the opening words St. Ignatius gives glory to God "that it hath been vouchsafed to me to see thy face". It seems hardly safe to infer, with Pearson and Lightfoot, from these words that the two had never met before.
The Epistle of St. Polycarp to the Philippians
The Epistle of St. Polycarp was a reply to one from the Philippians, in which they had asked St. Polycarp to address them some words of exhortation; to forward by his own messenger a letter addressed by them to the Church of Antioch; and to send them any epistles of St. Ignatius which he might have. The second request should be noted. St. Ignatius had asked the Churches of Smyrna and Philadelphia to send a messenger to congratulate the Church of Antioch on the restoration of peace; presumably, therefore, when at Philippi, he gave similar instructions to the Philippians. This is one of the many respects in which there is such complete harmony between the situations revealed in the Epistles of St. Ignatius and the Epistle of St. Polycarp, that it is hardly possible to impugn the genuineness of the former without in some way trying to destroy the credit of the latter, which happens to be one of the best attested documents of antiquity. In consequence some extremists, anti-episcopalians in the seventeenth century, and members of the Tubingen School in the nineteenth, boldly rejected the Epistle of Polycarp [the only credible intellectual position, if you honestly reject Ignatius, but one which no individual today impugning Ignatius makes--p2brc.] Others tried to make out that the passages which told most in favour of the Ignatian epistles were interpolations.
These theories possess no interest now that the genuineness of the Ignatian epistles has practically ceased to be questioned. The only point raised which had any show of plausibility (it was sometimes used against the genuineness, and sometimes against the early date of St. Polycarp's Epistle) was based on a passage in which it might at first sight seem that Marcion was denounced: "For every one who does not confess that Jesus Christ has come in the flesh is antichrist; and whosoever does not confess the testimony of the cross, is a devil, and whosoever perverteth the oracles of the Lord (to serve) his own lusts, and saith there is neither resurrection nor judgment, this man is a first-born of Satan." St. Polycarp wrote his epistle before he had heard of St. Ignatius' martyrdom. Now, supposing the passage just quoted to have been aimed at Marcion (whom, on one occasion, as we shall presently see, St. Polycarp called to his face "the first-born of Satan"), the choice lies between rejecting the epistle as spurious on account of the anachronism, or bringing down its date, and the date of St. Ignatius' martyrdom to A.D. 130-140 when Marcion was prominent. Harnack seems at one time to have adopted the latter alternative; but he now admits that there need be no reference to Marcion at all in the passage in question (Chronologie, I, 387-8). Lightfoot thought a negative could be proved. Marcion, according to him, cannot be referred to because nothing is said about his characteristic errors, e.g., the distinction between the God of the Old and the God of the New Testament; and because the antinomianism ascribed to "the first-born of Satan" is inapplicable to the austere Marcion (Lightfoot, St. Ignatius and St. Polycarp, I, 585; all references to Lightfoot (L), unless otherwise stated, will be to this work).
When Lightfoot wrote it was necessary to vindicate the authenticity of the Ignatian epistles and that of St. Polycarp. If the former were forgeries, the latter, which supports - it might almost be said presupposes- them, must be a forgery from the same hand. But a comparison between Ignatius and Polycarp shows that this is an impossible hypothesis. The former lays every stress upon episcopacy, the latter does not even mention it. The former is full of emphatic declarations of the doctrine of the Incarnation, the two natures of Christ, etc. In the latter these matters are hardly touched upon. "The divergence between the two writers as regards Scriptural quotations is equally remarkable. Though the seven Ignatian letters are many times longer than Polycarp's Epistle, the quotations in the latter are incomparably more numerous, as well as more precise, than in the former. The obligations to the New Testament are wholly different in character in the two cases. The Ignatian letters do, indeed, show a considerable knowledge of the writings included in our Canon of the New Testament; but this knowledge betrays itself in casual words and phrases, stray metaphors, epigrammatic adaptations, and isolated coincidences of thought...On the other hand in Polycarp's Epistle sentence after sentence is frequently made up of passages from the Evangelical and Apostolic writings...But this divergence forms only part of a broader and still more decisive contrast, affecting the whole style and character of the two writings. The profuseness of quotations in Polycarp's Epistle arises from a want of originality...On the other hand the letters of Ignatius have a marked individuality. Of all early Christian writings they are pre-eminent in this respect" (op.cit., 595-97).