Which Bible text did they read: the Hebrew or the Greek Septuagint?

Justin Martyr c. 100–c. 165 ADDialogue with Trypho 71.1–2

I do not trust your teachers, who refuse to admit that the translation made by the seventy elders at the court of Ptolemy, king of Egypt, is correct, and try to make their own. I want you to know that they have entirely removed many passages from the translation made by the elders at Ptolemy's court, passages that show plainly that this very man who was crucified was proclaimed as God and man, crucified and dying.

By the second century Jewish scholars made new Greek translations (Aquila and others). Christians held to the older Septuagint, especially for Isaiah 7:14, "the virgin shall conceive".

Irenaeus of Lyons c. 130–c. 202 ADAgainst Heresies 3.21.2, preserved in Eusebius, Church History 5.8.11–14

Ptolemy son of Lagus, eager to adorn the library he had built at Alexandria with the writings of all peoples, asked the people of Jerusalem to have their Scriptures turned into Greek. They sent seventy elders, the most skilled in the Scriptures and in both languages. Ptolemy, wishing to test them and fearing they might agree together to hide the truth of the Scriptures in the translation, separated them and told them all to write the same translation, and did this for all the books.

Irenaeus goes on: they all agreed word for word, so the translation was by God's inspiration. He uses this against new translations that read "young woman" in Isaiah 7:14.

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's Hexapla set the Hebrew, a Greek transliteration and four Greek translations side by side. He kept the Septuagint for the churches but marked where it differs from the Hebrew.

Jerome c. 340/345–420 ADPreface to Samuel and Kings end of the preface

So read first my Samuel and Kings: mine, I say, mine. Whatever we have learned and hold by often translating and carefully correcting is ours. And when you understand what you did not know before, think me a translator if you are grateful, or a paraphraser if you are not, though I am not aware that I have changed anything from the Hebrew truth. If you doubt it, read the Greek and Latin copies and compare.

Jerome's new Latin from the Hebrew became the Vulgate. Augustine worried it would divide the churches from the Septuagint they knew.

Council of Trent (Catholic)Session 4 Decree on the Edition and Use of the Sacred Books

...that the said old and vulgate edition, which, by the lengthened usage of so many years, has been approved of in the Church, be, in public lectures, disputations, sermons and expositions, held as authentic

Trent made Jerome's Latin the authentic text for public use in the Latin Church, without condemning the Hebrew or Greek. The Orthodox still read the Septuagint.

Westminster Confession (Reformed)Westminster Confession of Faith 1.8

The Old Testament in Hebrew (which was the native language of the people of God of old), and the New Testament in Greek (which, at the time of the writing of it, was most generally known to the nations), being immediately inspired by God, and, by his singular care and providence, kept pure in all ages, are therefore authentical; so as, in all controversies of religion, the church is finally to appeal unto them.

Protestant translations since the Reformation follow the Hebrew for the Old Testament.