Within the Jewish community, literacy was high as a result of the desire to read the Hebrew bible. And even if it wasn't, there was strong engagement with the text which boys were taught by rote; the same sort of approach is seen amongst Muslims to this day.

With the Gentile Christian community, the words of Jesus appear to have been circulated, translated into Greek, from a very early stage. Again a tradition of learning stories for oral transmission is well attested.

In both cases, whilst the message of Jesus may not 'come from a book', it does comes by words that were preached and received by the power of the Holy Spirit.

And consider Jesus' teaching on divorce; rather than speak on his own authority, he bases his teaching on the Hebrew Bible:

Some Pharisees came to him to test him. They asked, “Is it lawful for a man to divorce his wife for any and every reason?”

4 “Haven’t you read,” he replied, “that at the beginning the Creator ‘made them male and female,’ 5 and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’? 6 So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.”

7 “Why then,” they asked, “did Moses command that a man give his wife a certificate of divorce and send her away?”

8 Jesus replied, “Moses permitted you to divorce your wives because your hearts were hard. But it was not this way from the beginning. 9 I tell you that anyone who divorces his wife, except for sexual immorality, and marries another woman commits adultery.”

We have no authority to be less faithful to the teaching of the whole of the bible.