The Early Church Fathers on various topics: This was a 3700 hour project which included going through 22896 pages of the 38 volume set called Ante Nicene, Nicene, Post Nicene Fathers. I compiled 255 pages of quotes showing that the Early Church was always and completely Catholic. All of these quotes can be verified and found from the source which is free online.
These are Catholics who are quoting Paul's "Another Gospel" showing that another gospel is that which is not taught by the original faith which is Catholic Christianity.
"I marvel that ye are so soon removed from Him that called you into the grace of Christ, unto another gospel: which is not another; but there be some that trouble you, and would pervert the Gospel of Christ. But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed."
Archelaus Acts of the Disputation with the Heresia par 34
For if he that cometh preacheth another Christ, whom we have not preached, or if ye receive another Spirit, which we have not received, or another gospel, which ye have not accepted, ye might well bear with him. For I suppose that I did nothing less for you than the other apostles."
Athanasius Ad Episcopis Aegypti et Libyae (296-373 ad)
For even if, now that he is a creeping serpent, he shall transform himself into an angel of light, yet his deception will not profit him; for we have been taught that 'though an angel from heaven preach unto us any other gospel than that we have received, he is anathema
Athanasius Disposition of Arius par 5-6 (296-373 ad)
While Paul, who was taught these things by our Saviour, wrote that "in the latter times some shall depart from the sound faith, giving heed to seducing spirits and doctrines of devils, which reject the truth ." [+] 6. Since then our Lord and Saviour Jesus, Christ has instructed us by His own mouth, and also hath signified to us by the Apostle concerning such men, we accordingly being personal witnesses of their impiety, have anathematized, as we said, all such, and declared them to be alien from the Catholic Faith and Church.
Athanasius De Decretis ch 2.5 (296-373 ad)
"If anyone preach any other Gospel unto you than that ye have received, let him be anathema, as I have said, so say I again. If even we, or an Angel from heaven should preach unto you any other Gospel than that ye have received, let him be anathema." Since then the Apostle thus speaks, let these men either anathematise Eusebius and his fellows, at least as changing round and professing what is contrary to their subscriptions; or, if they acknowledge that their subscriptions were good, let them not utter complaints against so great a Council. But if they do neither the one nor the other, they are themselves too plainly the sport of every wind and surge, and are influenced by opinions, not their own, but of others, and being such, are as little worthy of deference now as before, in what they allege.
Hilary of Poitiers On the Trinity book 10 par 2 (300-367 ad)
But the Spirit saith expressly that in the last days some shall away from the faith, giving heed to seducing spirits, and doctrines of devils through the hypocrisy of lying talk. What advancement of doctrine is it to discover what one fancies, and not what one ought to learn? Or what piety in doctrine is it not to desire what one ought to learn, but to heap up doctrine after our desires? But this is what the promptings of seducing spirits supply. They confirm the falsehoods of pretended godliness, for a canting hypocrisy always succeeds to defection from the faith: so that at least in word the reverence is retained, which the conscience has lost. Even that pretended piety they make impious by all manner of lies, violating by schemes of false doctrine the sacredness of the faith: for they pile up doctrines to suit their desires, and not according to the faith of the Gospel. They delight, with an uncontrollable pleasure, to have their itching ears tickled by the novelty of their favourite preaching; they estrange themselves utterly from the hearing of the truth, and surrender themselves entirely to fables: so that their incapacity for either speaking or understanding the truth invests their discourse with what is, to them, a semblance of truth.
Gregory of Nyssa On Virginity ch 8 (325-386 ad)
LET no one think however that herein we depreciate marriage as an institution. We are well aware that it is not a stranger to God's blessing. But since the common instincts of mankind can plead sufficiently on its behalf, instincts which prompt by a spontaneous bias to take the high road of marriage for the procreation of children, whereas Virginity in a way thwarts this natural impulse, it is a superfluous task to compose formally an Exhortation to marriage. We put forward the pleasure of it instead, as a most doughty champion on its behalf. It may be however, notwithstanding this, that there is some need of such a treatise, occasioned by those who travesty the teaching of the Church. Such persons(8) "have their conscience seared with a hot iron," as the Apostle expresses it; and very truly too, considering that, deserting the guidance of the Holy Spirit for the "doctrines of devils,"
Basil Letter 199 par 47 (329-379 ad)
Their heresy is, as it were, an offshoot of the Marcionites, abominating, as they do, marriage, refusing wine, and calling God's creature polluted. We do not therefore receive them into the Church, unless they be baptized into our baptism.
Jerome Letter 95 par 2 (347-420 ad)
Being informed, then, by a letter of the aforesaid bishop, we inform your holiness that we in like manner who are set in the city of Rome in which the prince of the apostles, the glorious Peter, first founded the church and then by his faith strengthened it; to the end that no man may contrary to the commandment read these books which we have mentioned, have condemned the same; and have with earnest prayers urged the strict observance of the precepts which God and Christ have inspired the evangelists to teach. We have charged men to remember the words of the venerable apostle Paul, prophetic and full of warning:--"if any than preach any other gospel unto you than that which we have preached unto you, let him be accursed."
Jerome Letter 69 par 5 (347-420 ad)
The text quoted by the objector, "a bishop must be the husband of one wife," admits of quite another explanation. The apostle came of the Jews and the primitive Christian church was gathered out of the remnants of Israel. Paul knew that the Law allowed men to have
children by several wives, and was aware that the example of the patriarchs had made polygamy familiar to the people.
John Chrysostom Homily 12 on First Timothy (347-407 ad)
"Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of demons; speaking lies in hypocrisy; having their conscience seared with a hot iron; forbidding to marry, and commanding to abstain from meats, which God hath created to be received with thanksgiving of them which believe and know the truth."
As those who adhere to the faith are fixed on a safe anchor, so those who fall from the faith can nowhere rest; but after many wanderings to and fro, they are borne at last into the very gulf of perdition. And this he had shown before, saying, that some had and now he says, "Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits." This is said of the Manichaeans, the Encratites, and the Marcionites, and the whole of their tribe, that they should hereafter depart from the faith. Seest thou that this departure from the faith is the cause of all the evils that follow!
John Chrysostom Homily 27 on First Corinthians (347-407 ad)
"If even an angel preach any other gospel unto you than that ye have received, let him be accursed;" would be justified by the law, ye are fallen away from grace." (Gal. v. 4.)
Augustine Reply to Faustus the Manichean Book 31 par 2 (354-430 ad)
But there can be no excuse for you in the face of Paul's announcement, that there is nothing which is not pure, and that abstinence from certain food is the doctrine of devils, and that those who think anything defiled are polluted in their mind, if you not only abstain, as we have said, but make a merit of it,
Augustine Reply to Faustus the Manichean Book 16 par 25 (354-430 ad)
Moses no doubt knew in the spirit of prophecy, and from what he himself heard from God. that many heretics, would arise to teach errors of all kinds against the doctrine of Christ, and to preach another Christ than the true Christ. For the true Christ is He that was foretold in the prophecies uttered by Moses himself, and by the other holy men of that nation. Moses accordingly commanded that whoever tried to teach another Christ should be put to death. In obedience to this command, the voice of the Catholic Church, as with the spiritual two-edged sword of both Testaments, puts to death all who try to turn us away from our God, or to break any of the commandments
Vincent of Lerins Commonitory par 63 (390-450 ad)
There are innumerable instances of this kind, which for brevity's sake, pass over; by all of which, however, it is manifestly and clearly shown, that it is an established law, in the case of almost all heresies, that they evermore delight in profane novelties, scorn the decisions of antiquity, and, through oppositions of science falsely so called, make shipwreck of the faith. On the other hand, it is the sure characteristic of Catholics to keep that which has been committed to their trust by the holy Fathers, to condemn profane novelties, and, in the apostle's words, once and again repeated, to anathematize every one who preaches any other doctrine than that which has been received.(
Theodoret Ecclesial History book 2 ch 6 (393-457 ad)
For the apostle's precept enjoins, if any one should bring to you another gospel than that which ye have received, let him be accursed(15). Command that no one hold communion with them; for light can have no fellowship with darkness. Keep far off from them; for what concord has Christ with Belial? Be careful, beloved brethren, that you neither write to them nor receive their letters. Endeavour, beloved brethren and fellow-ministers, as though present with us in spirit at the council, to give your hearty consent to what is enacted, and affix to it your written signature, for the sake of preserving unanimity of opinion among all our fellow-ministers throughout the world (goes on to say who he is talking about “Valens and Ursacius have, like two vipers brought forth by an asp, proceeded from the Arian heresy.”)