Mary remained ever-virgin after the birth of Christ

But, as appears, many even down to our own time regard Mary, on account of the birth of her child, as having been in the puerperal state, although she was not. For some say that, after she brought forth, she was found, when examined, to be a virgin.

Now such to us are the Scriptures of the Lord, which gave birth to the truth and continue virgin, in the concealment of the mysteries of the truth. “And she brought forth, and yet brought not forth,” says the Scripture; as having conceived of herself, and not from conjunction. Wherefore the Scriptures have conceived to Gnostics; but the heresies, not having learned them, dismissed them as not having conceived.

Clement of Alexandria
The Stromata, Book VII
Chapter XVI

Heretics celebrate Eucharist with bread and water

Scripture manifestly applying the terms bread and water to nothing else but to those heresies, which employ bread and water in the oblation, not according to the canon of the Church. For there are those who celebrate the Eucharist with mere water.

Clement of Alexandria
The Stromata, Book I
Chapter XIX

Heresies are judged against tradition

The dogmas taught by remarkable sects will be adduced; and to these will be opposed all that ought to be premised in accordance with the profoundest contemplation of the knowledge, which, as we proceed to the renowned and venerable canon of tradition, from the creation of the world, will advance to our view; setting before us what according to natural contemplation necessarily has to be treated of beforehand, and clearing off what stands in the way of this arrangement. So that we may have our ears ready for the reception of the tradition of true knowledge;

Clement of Alexandria
The Stromata, Book I
Chapter I

The Church is the paradise of Eden

…but to flee to the Church, and be brought up in her bosom, and be nourished with the Lord’s Scriptures. For the Church has been planted as a garden (paradisus) in this world; therefore says the Spirit of God, “Thou mayest freely eat from every tree of the garden,” that is, Eat ye from every Scripture of the Lord; but ye shall not eat with an uplifted mind, nor touch any heretical discord.

Irenaeus of Lyons
Against Heresies, Book V
Chapter XX

No reformation of the Church through schism will ever make up for it’s mischief

He shall also judge those who give rise to schisms, who are destitute of the love of God, and who look to their own special advantage rather than to the unity of the Church; and who for trifling reasons, or any kind of reason which occurs to them, cut in pieces and divide the great and glorious body of Christ, and so far as in them lies, [positively] destroy it,—men who prate of peace while they give rise to war, and do in truth strain out a gnat, but swallow a camel. For no reformation of so great importance can be effected by them, as will compensate for the mischief arising from their schism. He shall also judge all those who are beyond the pale of the truth, that is, who are outside the Church; but he himself shall be judged by no one.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXXIII

Gatherings outside of Apostolic succession should be viewed with suspicion

Wherefore it is incumbent to obey the presbyters who are in the Church,—those who, as I have shown, possess the succession from the apostles; those who, together with the succession of the episcopate, have received the certain gift of truth, according to the good pleasure of the Father. But [it is also incumbent] to hold in suspicion others who depart from the primitive succession, and assemble themselves together in any place whatsoever, [looking upon them] either as heretics of perverse minds, or as schismatics puffed up and self-pleasing, or again as hypocrites, acting thus for the sake of lucre and vainglory.

…But those who cleave asunder, and separate the unity of the Church, [shall] receive from God the same punishment as Jeroboam did.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XXVI

 

Eucharistic heresies of Marcion and the Gnostics

For some, by maintaining that the Father is different from the Creator, do, when they offer to Him what belongs to this creation of ours, set Him forth as being covetous of another’s property, and desirous of what is not His own.

Those, again, who maintain that the things around us originated from apostasy, ignorance, and passion, do, while offering unto Him the fruits of ignorance, passion, and apostasy, sin against their Father, rather subjecting Him to insult than giving Him thanks.

Irenaeus of Lyons
Against Heresies, Book IV
Chapter XVIII

Ignoring heresy is not an option

Wherefore it shall not weary us, to endeavor with all our might to stretch out the hand unto them. Over and above what has been already stated, I have deferred to the following book, to adduce the words of the Lord; if, by convincing some among them, through means of the very instruction of Christ, I may succeed in persuading them to abandon such error, and to cease from blaspheming their Creator, who is both God alone, and the Father of our Lord Jesus Christ. Amen.

Irenaeus of Lyons
Against Heresies, Book III
Chapter XXV