How can we trust that the text was copied faithfully?

Dionysius of Corinth fl. c. 171 ADLetter, preserved in Eusebius, Church History 4.23.12

When brothers asked me to write letters, I wrote them. And the devil's apostles have filled them with weeds, taking some things out and adding others. Woe awaits them. So it is no wonder that some have set out to tamper even with the Lord's writings, since they have plotted against writings of lesser weight.

Dionysius was bishop of Corinth. The "Lord's writings" may mean the Gospels.

Irenaeus of Lyons c. 130–c. 202 ADOn the Ogdoad, preserved in Eusebius, Church History 5.20.2

I adjure you who copy this book, by our Lord Jesus Christ and by his glorious coming, when he comes to judge the living and the dead, that you compare what you have copied, and correct it carefully against this copy from which you copied it; and that you copy this oath likewise and put it in the copy.

Irenaeus asks this for his own book. Eusebius quotes it as a model of the care early Christians took in copying.

Tertullian c. 155–c. 220 ADPrescription against Heretics 36.1–2

Come now, you who want to exercise your curiosity better in the business of your salvation: go through the apostolic churches, where the very chairs of the apostles still preside in their places, where their own authentic letters are read, sounding the voice and showing the face of each. Achaia is nearest you: you have Corinth. If you are not far from Macedonia, you have Philippi and the Thessalonians. If you can cross into Asia, you have Ephesus. If you are near Italy, you have Rome.

"Authentic letters" may mean the originals or trustworthy copies; scholars disagree. Either way, the letters were read in the churches that received them.

Anonymous writer against ArtemonLittle Labyrinth, preserved in Eusebius, Church History 5.28.15–17

So they laid hands on the divine Scriptures without fear, saying they had corrected them. That I am not slandering them, anyone who wishes can learn: if someone collects their copies and examines them against one another, he will find them very much at odds. Asclepiades's copies will not agree with Theodotus's ... what they call corrected copies, that is, corrupted ones.

The test is comparison: the altered copies disagree with each other and with the churches' copies. The writer may be Hippolytus.

Origen of Alexandria c. 185–c. 254 ADLetter to Africanus 7–9

I say this not from reluctance to search the Jewish Scriptures and compare all ours with theirs and see the differences. If it is not boastful to say it, we have done this at length as far as we could, training our minds on all the editions and their differences, while giving more attention to the translation of the Seventy, so as not to seem to forge anything for the churches under heaven.

Origen spent years comparing copies and translations. His Hexapla is the first large critical edition of the Bible.

Origen of Alexandria c. 185–c. 254 ADCommentary on Matthew 15.14

But as things are, it is clear that the difference among the copies has become great, whether from the carelessness of some scribes, or from the wicked daring of some in correcting what was written, or from those who, in correcting, added or took away what seemed good to them.

Origen is weighing why Matthew's list of commandments to the rich young man includes "You shall love your neighbour as yourself" where the other Gospels differ. The Matthew commentary witness in the store holds only books 10 and 11, which is why the passage was not found there; it stands in book 15, and the text here is Migne PG 13 from Khazarzar's second Matthew PDF (books 12-17), fetched for this dossier. Origen goes on: for the Old Testament he had healed the disagreement by judging from the other translations, the work of the Hexapla.

Augustine of Hippo 354–430 ADOn Christian Teaching 2.14.21

A large number of translators also helps a great deal, when their copies are compared, examined and discussed, provided there is no falsehood. For those who want to know the divine Scriptures should first of all take care to correct their copies, so that uncorrected copies give way to corrected ones, at least when they come from the same kind of translation.

Augustine treats differences between copies as normal, and correcting them as the first task of a reader of Scripture.