Scripture cannot be properly interpreted outside the Apostolic churches

Our appeal, therefore, must not be made to the Scriptures; nor must controversy be admitted on points in which victory will either be impossible, or uncertain, or not certain enough. But even if a discussion from the Scriptures should not turn out in such a way as to place both sides on a par, (yet) the natural order of things would require that this point should be first proposed, which is now the only one which we must discuss: “With whom lies that very faith to which the Scriptures belong. From what and through whom, and when, and to whom, has been handed down that rule, by which men become Christians?” For wherever it shall be manifest that the true Christian rule and faith shall be, there will likewise be the true Scriptures and expositions thereof, and all the Christian traditions.

Tertullian
Against Marcion
The Prescription Against Heretics
Chapter XIX

Heretics add and subtract from Scripture, and come up with new interpretations

Now this heresy of yours does not receive certain Scriptures; and whichever of them it does receive, it perverts by means of additions and diminutions, for the accomplishment of it own purpose; and such as it does receive, it receives not in their entirety; but even when it does receive any up to a certain point as entire, it nevertheless perverts even these by the contrivance of diverse interpretations. Truth is just as much opposed by an adulteration of its meaning as it is by a corruption of its text. Their vain presumptions must needs refuse to acknowledge the (writings) whereby they are refuted. They rely on those which they have falsely put together, and which they have selected, because of their ambiguity. Though most skilled in the Scriptures, you will make no progress, when everything which you maintain is denied on the other side, and whatever you deny is (by them) maintained. As for yourself, indeed, you will lose nothing but your breath, and gain nothing but vexation from their blasphemy.

Tertullian
Against Marcion
The Prescription Against Heretics
Chapter XVII

Heresies always arise out of a twisting of the Scriptures

When men, therefore, are not Christians even on their own admission, how much more (do they fail to appear such) to us! What sort of truth is that which they patronize, when they commend it to us with a lie?  Well, but they actually treat of the Scriptures and recommend (their opinions) out of the Scriptures! To be sure they do. From what other source could they derive arguments concerning the things of the faith, except from the records of the faith?

Tertullian
Against Marcion
The Prescription Against Heretics
Chapter XIV

Masses for the dead, fasting, kneeling and the sign of the cross during worship

As often as the anniversary comes round, we make offerings for the dead as birthday honors. We count fasting or kneeling in worship on the Lord’s day to be unlawful. We rejoice in the same privilege also from Easter to Whitsunday. We feel pained should any wine or bread, even though our own, be cast upon the ground.  At every forward step and movement, at every going in and out, when we put on our clothes and shoes, when we bathe, when we sit at table, when we light the lamps, on couch, on seat, in all the ordinary actions of daily life, we trace upon the forehead the sign.

If, for these and other such rules, you insist upon having positive Scripture injunction, you will find none. Tradition will be held forth to you as the originator of them, custom as their strengthener, and faith as their observer. That reason will support tradition, and custom, and faith, you will either yourself perceive, or learn from some one who has.

…This instances, therefore, will make it sufficiently plain that you can vindicate the keeping of even unwritten tradition established by custom; the proper witness for tradition when demonstrated by long-continued observance.

Tertullian
The Chaplet
Chapter III and IV

Tradition confirms what is not in scripture

And how long shall we draw the saw to and fro through this line, when we have an ancient practice, which by anticipation has made for us the state, i.e., of the question? If no passage of Scripture has prescribed it, assuredly custom, which without doubt flowed from tradition, has confirmed it. For how can anything come into use, if it has not first been handed down?

Tertullian
The Chaplet
Chapter III

Traditions of the Septuagint

Now in ancient times the people we call Jews bare the name of Hebrews, and so both their writings and their speech were Hebrew. But that the understanding of their books might not be wanting, this also the Jews supplied to Ptolemy; for they gave him seventy-two interpreters—men whom the philosopher Menedemus, the well-known asserter of a Providence, regarded with respect as sharing in his views.

Tertullian
Apology
Chapter XVIII

Traditions of Peter, Mark, Paul, Luke and Jude

“Marcus, my son, saluteth you.” Mark, the follower of Peter, while Peter publicly preached the Gospel at Rome before some of Cæsar’s equites, and adduced many testimonies to Christ, in order that thereby they might be able to commit to memory what was spoken, of what was spoken by Peter, wrote entirely what is called the Gospel according to Mark. As Luke also may be recognised by the style, both to have composed the Acts of the Apostles, and to have translated Paul’s Epistle to the Hebrews.

Jude, who wrote the Catholic Epistle, the brother of the sons of Joseph, and very religious, whilst knowing the near relationship of the Lord, yet did not say that he himself was His brother.

Clement of Alexandria
The Stromata, Book VIII
Chapter VIII

The Christian life is deeds and words in the tradition of the Lord

Our Gnostic then alone, having grown old in the Scriptures, and maintaining apostolic and ecclesiastic orthodoxy in doctrines, lives most correctly in accordance with the Gospel, and discovers the proofs, for which he may have made search (sent forth as he is by the Lord), from the law and the prophets. For the life of the Gnostic, in my view, is nothing but deeds and words corresponding to the tradition of the Lord.

Clement of Alexandria
The Stromata, Book VI
Chapter XVI

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

Unwritten Apostolic tradition has been handed down

Now that the Savior has taught the apostles, the unwritten rendering of the written [Scripture] has been handed down also to us, inscribed by the power of God on hearts new, according to the renovation of the book.

Clement of Alexandria
The Stromata, Book VI
Chapter XV