When pieces of tradition are quoted and used in early Christian authors, their function in the life of the community is usually maintained. Indeed, it may not even be necessary to refer to them as traditions related to Jesus. This is most clearly the case in Paul's allusions to sayings of Jesus in Romans 12–14, in 1 Peter, and in the Epistle of James. As far as writings such as 1 Clement, Barnabas, and the letters of Ignatius are concerned, use of sayings of Jesus and allusions to them would seem to be natural continuations of this practice, whether or not Jesus is explicitly mentioned as an authority. Sayings of Jesus were known because they had been established as parts of a Christian catechism; the passion narrative was known because it was embedded into the Christian liturgy. (Koester 1994:297; emphasis added)
The entire perspective informing this paragraph seems absurd to me. Notice how Koester privileges a tradition's "function in the life of the community" over its significance as a "tradition related to Jesus." When, for example, a Christian text says something like, "Bless those who persecute you; bless and do not curse" (Rom. 12.14), that text is clearly performing certain functions among the Christian community. However, when another text narrates Jesus saying something to this effect (e.g., Matt. 5.43–48), the latter text is less authentic because its communal function has been muted. (That is, in Romans 12 Paul is overtly trying to persuade the Roman Christians to think and behave a certain way; in Matthew, the Evangelist may be endorsing certain patterns of thought and behavior, but his endorsement is covert, masked by the overt aim of narrating the life and teaching of Jesus.) I can't help but wonder where Koester thinks Jesus' followers procured the traditions that were at work in their communities if not from the teachings of the historical Jesus. I'm not even claiming that such traditions had to actually stem from the historical Jesus; only that the tradition's significance as a "tradition related to Jesus" must be more salient than any purported "function in the life of the community."
But even more to the point, I don't understand how Koester can claim that Jesus' teachings "were known because they had been established as parts of a Christian catechism," or that "the passion narrative was known because it was embedded into the Christian liturgy." Koester is clearly a fan of putting carts before horses. Where does Koester think Christian catechisms and liturgy came from if not from the sayings and passion of Jesus, among other things? Instead, the teachings of Jesus and the passion narrative were known because they were foundational to the beliefs and practices of Jesus' followers. As a result, their catechisms and liturgy reflected his teachings and passion. While the catechisms and liturgy might have been vehicles of the traditions of Jesus' teachings and passion, they were not the motivation or reason for knowing these traditions. Otherwise, Koester needs to explain what motivated the church's catechisms and liturgy.