Early church writers and theologians often commented on the Old Testament as "sacred scripture" and Christian communities which were most likely predominately gentile preserved the Old Testament (we have quite a few 2nd and 3rd century fragments; a 1st century Psalms manuscript is probably the earliest of Christian provenance.) Our earliest Bibles (e.g. Codex Sinaiticus) included the OT and they continue to do today.
In 2 Timothy 3:16 the author exhorts his audience to continue in the practice they have learnt from the beginning:
"But as for you, continue in what you have learned and have firmly believed, knowing from whom you learned it and how from childhood you have been acquainted with the sacred writings, which are able to make you wise for salvation through faith in Christ Jesus. All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be competent, equipped for every good work." 2 Tim 3:14-17 (ESV)
Clement of Rome refers to the Old Testament as "scripture" and “sacred scripture” and uses it as the source for much of his theology and teaching. A few random examples from his Epistle to the Corinthians (1 Clement) from the end of the first century:
- He uses Psalms and Job to explain his theology of resurrection (26)
- At 28 he states that God sees all things, quoting Psalm 139:7-10
- At 45 he gives a high theology of the Old Testament exhorting the Corinthians to, "Look carefully into the Scriptures, which are the true utterances of the Holy Spirit."
- At 53 he tells us that the Corinthians he is writing to "understand well the Sacred scriptures." He quotes and paraphrases Exodus 32 and Deuteronomy 9.In the latter half of the 2nd century, Justin Martyr recounts an engagement with a Jew named Trypho. They discuss the Old Testament, issues of the 'New Covenant' (Jeremiah 31:31-32/Dialogue 11), the nature of the law and ritual before Moses, etc. Justin wrote:
For these words have neither been prepared by me, nor embellished by the art of man; but David sung them, Isaiah preached them, Zechariah proclaimed them, and Moses wrote them. Are you acquainted with them, Trypho? They are contained in your Scriptures, or rather not yours, but ours. For we believe them; but you, though you read them, do not catch the spirit that is in them.(Dialogue 29)
We also know that Greek and Latin speaking Christians learnt Hebrew in order to properly understand the Old Testament. It should be noted that most Jews and Christians of the time thought this was unnecessary as it was believed that the Septuagint Greek translation was an authoritative translation. Origen in the third century learnt Hebrew to study the OT indetail. We have surviving copies of his homilies on Genesis, Exodus, Leviticus and Numbers. We also know that the Syriac translation of the OT known as the Peshitta was used by gentile Christians such as Aphraat and Ephrem.
If Luke was in fact a gentile his intimate knowledge of the Septuagint is noteworthy. As there is much I have not read of the early Church Fathers I can only say so much.