Material passed on from generation to generation by word of mouth. There was a long process of gestation before books of the OT reached their final written form. In ancient Israel pieces of poetry, stories of heroes, legends about founding figures, and songs of celebration circulated amongst groups and tribes. Repeatedly accounts of the momentous events of the Exodus and the wilderness journeys were transmitted to descendants and these were followed later by accounts of the early kings and the prophets; they all gathered embellishments and reinterpretations in the process. Then written collections were assembled—of laws, of prophetic utterances, stories about particular individuals. These writings in turn became the raw material for the recognizable written sources which can now be identified in the Pentateuch. Some modern scholars, especially in Scandinavia, where comparisons have been made with the Norse epics, have argued for a very long period of oral tradition; but it is more usually held that written sources go back to about the 10th cent. BCE.
The NT also had a period, much shorter, during which stories about Jesus and the first Christians were passed from person to person, and group to group, as the needs of teaching and controversy required. It meant that much material that was not needed simply perished and that which survived has passed through the modifications, interpretations, and theological reflections that the occasions demanded, and as naturally used during assemblies for worship. Preachers would choose items from a stock of memories, but with that freedom to elaborate controlled by authority figures. Sayings of Jesus were assembled and may have been strung together and used when the gospels were written. Matthew and Luke contain parables and shorter sayings of Jesus in their gospels, though their original contexts may have been lost. Mark compiled a Passion narrative with great care, to which he attached the eschatological predictions and affirmations of ch. 13, and prefaced all this with a rapid account of Jesus' journeys, conflicts, choice of disciples, miracles, and parables expounding and proclaiming the advent of the kingdom of God.
Oral tradition was not a Christian innovation. It was an established medium of Jewish scribes and rabbis. It was necessary to update the written injunctions of the Torah, so there were many legal stipulations ( halakah) and homiletic expositions of the OT narratives ( haggadah), and this oral transmission continued for several centuries until finally established in the Mishnah.