The following is the second chapter of Daillé's excellent work on the right use of the fathers. (see the contents post for more background)
Reason II. — Those writings which we have of the Fathers of the first centuries, treat of matters far different from the present controversies in religion.
But suppose that neither the want of books in the first three centuries, nor yet the abundance of them in the three following, should produce these inconveniences; it will nevertheless be very hard to discover from them what the opinion of their authors has been concerning those points of the Christian religion now controverted. For the matters whereof they treat are of a very different nature; these authors, according as the necessity of the times required, employing themselves either in justifying the Christian religion, and vindicating it from the aspersion of such crimes, wherewith it was most falsely and injuriously charged; or else in laying open to the world the absurdity and impiety of Paganism; or in convincing the hardhearted Jews, or in confuting the prodigious fooleries of the heretics of those times; or in exhortations to the faithful to patience and martyrdom; or in expounding some certain passages and portions of the Holy Scripture: all which things have very little concern with the controversies of these times; of which they never speak a syllable, unless they accidentally or by chance let a word drop from them toward this side or that side, yet without the least thought of us or of our controversies; although both the one and the other party sometimes light upon passages, wherein they conceive they have discovered their own opinions clearly delivered, though in vain for the most part, and without ground: precisely as he did, who on hearing the ringing of bells, thought they perfectly sounded out what he in his own thoughts had fancied. Justin Martyr and Tertullian, Theophilus and Lactantius, Clemens and Arnobius, show the heathen the vainness of their religion, and of their gods; and that Jupiter and Juno were but mortals, and that there is but one only God, the Creator of heaven and earth. Irenaeus bends his whole forces against the monstrous opinions of Basilides, the Valentinians, and other Gnostics, who were the inventors of the most chimerical divinity that ever came into the fancy of man. Tertullian also lashes them, as they well deserve; but he especially takes Marcion, Hermogenes, Apelles, Praxeas, and others to task, who maintained that there were two Gods, or two principles, and confounded the persons of the Father and the Son. Cyprian is wholly upon the discipline and the virtues of the Christian Church. Arius, Macedonius, Eunomius, Photinus, Pelagius, and afterwards Nestorius and Eutyches, made work for the Fathers of the fourth and fifth centuries.
The blasphemies of these men against the person or the natures of our Savior Christ, or against the Holy Ghost and his grace, which have now of a long time lain buried and forgotten, were the matters controverted in those times, and the subject of the greatest part of the books then written, that have come to our hands. What relation has anything of all this to the doctrines of transubstantiation, and the adoration of the Eucharist, or the monarchy of the Pope, or the necessity of auricular confession, or the worshiping of images, and similar points, which are those of the present controversies, and which none of the ancients have treated expressly and by design, or perhaps ever so much as thought of? It is very true indeed, that the silence of these Fathers in these points, which some set so much value by, is not wholly mute, and perhaps also it may pass for a very clear testimony, but certainly not on their side who maintain them affirmatively. But, however, this is a most certain truth, that throughout the whole body of the genuine writings of these Fathers, you shall not meet with anything expressly urged either for or against the greatest part of these opinions. I shall most willingly confess, that the belief of every wise man makes up but one entire body, the parts whereof have a certain correspondence and relation to each other, to such a degree that a man may be able by those things which he delivers expressly, to give a guess what his opinion is concerning other things of which he says nothing; it being utterly improbable that he should maintain any position which shall manifestly clash with his other tenets, or that he should reject anything that necessarily follows upon them. But, besides, this manner of disputation presupposes that the belief of the ancient Fathers is uniform, no one position contradicting another, but having all its parts united, and depending one upon another, which indeed is very questionable, as we shall show elsewhere. Besides all this, I say it requires a quick discernment, which readily and clearly apprehends the connexions of each distinct point, an excellent memory to retain faithfully whatever positions the ancients have maintained, and a solid judgment free from all preoccupation, to compare them with the tenets maintained at this day. And the man who is endued with all these qualities I shall account the fittest to make profitable use of the writings of the Fathers, and the likeliest of any to search deeply into them. But the mischief is, that men so qualified are very rare and difficult to be found.
I shall add here, that if you will believe certain writers of the Church of Rome,[Gontery, Veron, and others] this method is vain and useless, as is also that which makes use of argumentation and reason; means which are insufficient, and unable (in the judgment of these doctors) to arrive at any certainty, especially in matters of religion. Their opinion is, that we are to rely upon clear and express texts only. Thus, according to this account, we shall not, if we be wise, believe that the Fathers held any of the aforenamed points, unless we can find them in express terms in their writings; that is to say, in the very same terms that we read them in the decrees and canons of the Council of Trent. Seeing then that, according to the opinion of these men, those testimonies only are to be received which are express, and likewise that of these points now controverted there is scarcely anything found expressly delivered by the Fathers, we may, in my opinion, very logically and reasonably conclude, that it is at least a very difficult if not impossible thing (according to these men) to come to the certain knowledge of the opinion of the ancients concerning the greatest part of the tenets of the Church of Rome, which are at this day rejected by the Protestants.
TFan's notes: Daillé's point here is a very important one to recognize. One of the key problems with appeals to the fathers on many points is that they have not necessarily considered the issue we are considering. The Scriptures have an enormous depth. There are many truths we can learn from them, and consequently not every Christian in every age has learned all of them.
Likewise, the human ability to come up with errors is ever-abounding. No one had ever heard of a place called "Purgatory," or a process of "transubstantiation," during the earliest centuries of the church, for example. Until such an error was invented, the fathers could not explicitly reject the place or the process by name.
Furthermore, while we would like to assume a consistency in the fathers, such that they held not only to their stated views but to the logical conclusions of those views, that's a big assumption. After all, don't we know our own theological opponents today who do not hold to the logical implications of their own views?
Daillé also highlights the fact that Roman Catholics are themselves divided as to how to treat the fathers. Some he identifies would like to count the fathers' testimony only when they speak explicitly on the topic. Others are willing to consider the implications of what the fathers have to say. If the former rule is applied, virtually nothing of the disputed matters in Trent can be supported, for the words used by Trent differ from those used by the fathers. Indeed, if that rule is applied, it may be practically impossible to determine what the fathers would think of any controversy today.