There is only one place in the gospels where Jesus ever speaks of a New Testament. It is here:
“This chalice which is poured out for you is the new covenant in my blood.” (Luke 22:20)
Paul, the first to write of the Institution of the Eucharist, relates the same incident with slightly different words: “This chalice is the new covenant in my blood.” (1 Cor 11:25). Indeed, none of the gospels give us exactly the same words of Institution, because both Paul and the Evangelists are normal people giving us the gist of the apostles’ living memory of that one-time event and not pretending to be human tape recorders.
What matters here is that the Church saw the “new diatheke” (that is, the new covenant/testament established by Jesus in fulfillment of the promise of Jeremiah 31:31) as his gift of himself, not as his gift of a book. The books we call the New Testament were given that name because they were read in close proximity to the celebration of the Eucharist and explained the origin and meaning of the sacrament being celebrated.
This is significant because that means the reading of those books was understood to be something of a cautiously guarded thing in the early Church. There were no Christian bookstores hoping for sales among pagans. There was no bound copy of the Bible for preachers to use as they stood on corners and hollered at passersby.
Indeed, although there was a general consensus about which should be the core books of the New Testament, there were some minor differences from one diocese to the next about some of the more peripheral books such as 2 Peter, Hebrews, James, and Revelation. In addition, some bishops took a shine to things like Clement’s letter to the Corinthians. Eventually (and with surprisingly little acrimony) things settled down over the course of about three centuries and by the mid-fourth century everybody was (pardon the pun) on the same page.
But here’s the thing: this means that in the early Church, the New Testament documents were, in the experience of most Christians, a thing you heard at the liturgy, not a thing you read for yourself and still less a thing you read to non-believers. That was because, as a rule, you couldn’t read, being a slave or similar illiterate. It was also because books existed to be read aloud. (Augustine remarks on the marvel that you could stand right next to Ambrose of Milan and still not hear him reading.)
Moreover, ordering up your own copy of the New Testament was like buying your own family space shuttle. Only the fabulously wealthy could do such a thing. The New Testament was a book of the Church, by the Church, for the Church and was intended for the bosom of the Church.
But most of all, the Eucharist and the books—and above all the Church herself in all her members—stood in perpetual risk of being burnt by their neighbours. So they were treated as guarded mysteries.
What that means for us is that the promises—and warnings—of the New Testament are meant first of all for our ears, not for Those Non-Christians Over There.
This is an important first principle we can easily forget. We can fall into the trap of imagining that when Jesus, for instance, tells the Parable of the Sower, he means we Christians are the Good Seed and everybody else is the seed on the path, or gobbled by birds, or choked by thorns. But the gospels themselves stress the point that the parables were intended for his disciples and not for Outsiders. We are the ones in danger of failing to heed the warning, “He who has ears to hear, let him hear” (Matthew 11:15).
Relatedly, Jesus repeatedly rebukes attempts at clubbiness by the disciples. He crushes their fantasies of being The Greatest and bids them be the least and serve the least, including the least among their non-Christian neighbors. He rebukes them for chasing away little children and calling down fire on Samaritans. Indeed, he makes the despised Samaritan outsider the hero of his parable.
The point is this: Jesus strongly discourages comparisons and superiority games among his disciples. “What is that to you? Follow me!” is his counsel. The New Testament is most profitably read, not as a guidebook to being better than Those People Over There, but as a guide for those who seek to be disciples of Jesus Christ without comparing ourselves to others.