St. Irenaeus (b. ca. 115-142), in his Against Heresies, confirms almost every core Catholic doctrine: Roman/papal supremacy, the sacrifice of the Mass, transubstantiation in the Eucharist, the continuation of the earthly priesthood, the crucial mission and authority of the institutional Church, Church authority over Scriptural interpretations, the existence and authority of unchangeable and unbroken apostolic Tradition and succession through bishops, the necessity of obedience (as well as repentance and forgiveness) for salvation, denunciations of schismatics, Mary as the new Eve and as a cause of our salvation, prayers for the dead, the activity of the saints, etc.
Moreover, his work is filled with citations of Scripture.
He begins the work by acknowledging the plausibility of the heresies:
Inasmuch as certain men have set the truth aside, and bring in lying words and vain genealogies, which, as the apostle says, “minister questions rather than godly edifying which is in faith,” and by means of their craftily-constructed plausibilities draw away the minds of the inexperienced and take them captive, [I have felt constrained, my dear friend, to compose the following treatise in order to expose and counteract their machinations.] These men falsify the oracles of God, and prove themselves evil interpreters of the good word of revelation…. By means of specious and plausible words, they cunningly allure the simple-minded to inquire into their system; but they nevertheless clumsily destroy them, while they initiate them into their blasphemous and impious opinions respecting the Demiurge; and these simple ones are unable, even in such a matter, to distinguish falsehood from truth.
How do these heretics rationalize their heresy? Like all heretics, they use Scripture and claim superior understanding or mystical assistance in its interpretation:
They tell us, however, that this knowledge has not been openly divulged, because all are not capable of receiving it, but has been mystically revealed by the Saviour through means of parables to those qualified for understanding it.
Protestants have all these “experts” in exegesis for rationalizing their interpretations. When “reason” fails (as it always does, for there are endless plausible interpretations in isolation from Church tradition/history), the mystical and arbitrary support of the “Holy Spirit” supplies certainty for them.
Such, then, is the account which they all give of their Pleroma, and of the formation of the universe, striving, as they do, to adapt the good words of revelation to their own wicked inventions. And it is not only from the writings of the evangelists and the apostles that they endeavour to derive proofs for their opinions by means of perverse interpretations and deceitful expositions: they deal in the same way with the law and the prophets, which contain many parables and allegories that can frequently be drawn into various senses, according to the kind of exegesis to which they are subjected. And others of them, with great craftiness, adapted such parts of Scripture to their own figments, lead away captive from the truth those who do not retain a steadfast faith in one God, the Father Almighty, and in one Lord Jesus Christ, the Son of God.
Heretics by no means avoid or flee from Scripture. They are eager to make Scripture fit within their preconceived, dogmatic, heretical theology and worldview. And it’s easily done by those of creative imagination and “craftiness.” For Scripture passages ”can frequently be drawn into various senses, according to the kind of exegesis to which they are subjected.”
How does St. Irenaeus know with certainty that the doctrines in question are heresy? He tells us the apostles did not “deliver” such doctrines to the Church:
Such, then, is their system, which neither the prophets announced, nor the Lord taught, nor the apostles delivered, but of which they boast that beyond all others they have a perfect knowledge. They gather their views from other sources than the Scriptures; and, to use a common proverb, they strive to weave ropes of sand, while they endeavour to adapt with an air of probability to their own peculiar assertions the parables of the Lord, the sayings of the prophets, and the words of the apostles, in order that their scheme may not seem altogether without support. In doing so, however, they disregard the order and the connection of the Scriptures, and so far as in them lies, dismember and destroy the truth. By transferring passages, and dressing them up anew, and making one thing out of another, they succeed in deluding many through their wicked art in adapting the oracles of the Lord to their opinions…. In like manner do these persons patch together old wives’ fables, and then endeavour, by violently drawing away from their proper connection, words, expressions, and parables whenever found, to adapt the oracles of God to their baseless fictions.
All heretics know they have to find support within Scripture, otherwise they would have no support whatsoever for their errors and/or lies, for the authority and unchanging tradition of the Church always stand firmly against them.
Irenaeus condemns the heretics and offers a conclusive coup de grace which proves they are heretics:
You see, my friend, the method which these men employ to deceive themselves, while they abuse the Scriptures by endeavouring to support their own system out of them. For this reason, I have brought forward their modes of expressing themselves, that thus you might understand the deceitfulness of their procedure, and the wickedness of their error….
But since what may prove a finishing-stroke to this exhibition is wanting, so that any one, on following out their farce to the end, may then at once append an argument which shall overthrow it, we have judged it well to point out, first of all, in what respects the very fathers of this fable differ among themselves, as if they were inspired by different spirits of error. For this very fact forms an a priori proof that the truth proclaimed by the Church is immoveable, and that the theories of these men are but a tissue of falsehoods.
The clearest indication that a certain denomination is heretical is that their leaders keep dividing and schisming among themselves. At first there was only Luther; then came Calvin, Zwingli, and a horde of others. Now there are innumerable heretical sects that have sprung from the seed of Luther’s Revolution. There are now many different synods or conventions within mainstream Protestant lines. In many cases, Christianity has been degraded into a private, individual religion, cut off from any resemblance to the original apostolic faith that proclaims the true Church is of one Mind and one Body.
So how is the Christian to know which exegesis of and approach to Scripture is correct? Irenaus tells us that Holy Tradition, handed down unchanged historically from the apostles and universally throughout the world, is our assurance:
The Church, though dispersed throughout the whole world, even to the ends of the earth, has received from the apostles and their disciples this faith: … [Irenaeus gives a creed]
…but may, in the exercise of His grace, confer immortality on the righteous, and holy, and those who have kept His commandments, and have persevered in His love, some from the beginning [of their Christian course], and others from [the date of] their repentance, and may surround them with everlasting glory.
The true faith is received from the Church, not mystically or rationally divined from Scripture by individual believers. Notice also that immortality and salvation are given to those who are obedient to God’s commandments, which is perseverance in His love. “Faith alone” in some purely abstract/mental sense is not enough.
As I have already observed, the Church, having received this preaching and this faith, although scattered throughout the whole world, yet, as if occupying but one house, carefully preserves it. She also believes these points [of doctrine] just as if she had but one soul, and one and the same heart, and she proclaims them, and teaches them, and hands them down, with perfect harmony, as if she possessed only one mouth. For, although the languages of the world are dissimilar, yet the import of the tradition is one and the same. For the Churches which have been planted in Germany do not believe or hand down anything different, nor do those in Spain, nor those in Gaul, nor those in the East, nor those in Egypt, nor those in Libya, nor those which have been established in the central regions of the world. But as the sun, that creature of God, is one and the same throughout the whole world, so also the preaching of the truth shines everywhere, and enlightens all men that are willing to come to a knowledge of the truth. Nor will any one of the rulers in the Churches, however highly gifted he may be in point of eloquence, teach doctrines different from these (for no one is greater than the Master); nor, on the other hand, will he who is deficient in power of expression inflict injury on the tradition. For the faith being ever one and the same, neither does one who is able at great length to discourse regarding it, make any addition to it, nor does one, who can say but little diminish it.
Truth does not change. The Christian faith could not change in the 2nd century AD. It was already complete and perfect. Moreover, the true Church preserves the faith in its purity and hands it on, nothing more or less. Thus any later deviations from the Holy Tradition of the Church are by definition heretical, including the man-made traditions of the Protestant Revolution which popped into existence in the 16th century.
It does not follow because men are endowed with greater and less degrees of intelligence, that they should therefore change the subject-matter [of the faith] itself, and should conceive of some other God besides Him who is the Framer, Maker, and Preserver of this universe, (as if He were not sufficient for them), or of another Christ, or another Only-begotten. But the fact referred to simply implies this, that one may [more accurately than another] bring out the meaning of those things which have been spoken in parables, and accommodate them to the general scheme of the faith….
God never intended for individual intelligence or reason in and of itself to determine doctrine authoritatively, not to mention change it.
…as these teachers who are destitute of truly divine wisdom maintain; while the Catholic Church possesses one and the same faith throughout the whole world, as we have already said.
It is the catholic, the universal, Church that possesses the true faith.