St Clement of Rome’s Letter to the Corinthians was written towards the end of the first century AD. Clement writes on behalf of the Roman church to the church of Corinth to settle a dispute about authority there.
The letter is full of citations of scripture but all from the Greek Septuagint, that is, the Old Testament. For a modern Christian this is surprising until you realize that the only parts of the New Testament in wide circulation by 95 AD would have been the letters of Paul. The Gospels were likely written by then but were not yet in general use. “Scripture” for a first century Christian meant the Old Testament.
The Gospel therefore would have been communicated in preaching, ritual, and catechetical formulas. This is why the main concern of late first and early second century Christian letters is respect for presbyters and bishops: without a text to refer to, the way to keep the story straight was to have approved teachers.
Such heavy reliance on the Septuagint also explains why early Christian communities were so tempted to join Judaism since they were reading Jewish scriptures and history all the time.
Around the year 115 St Ignatius of Antioch spoke about the problem this way:
When I heard some people saying, “If I don’t find it in the original documents, I don’t believe it in the gospel,” I answered them, “But it is written there.” They retorted, “That’s just the question.” To my mind it is Jesus Christ who is the original documents. The inviolable archives are his cross and death and his resurrection and the faith that came by him.
By “original documents” he means the Septuagint, and by “the gospel” he does not mean a book, but the preaching of the church. The people he was arguing with were demanding his assertions about Christ be proven from the Old Testament. Ignatius’ response is that the witness of the church about Christ is the point of reference for understanding the Old Testament, not the other way around.