The parables of Jesus were all the rage in the second century. Gnostic teachers like Valentinus hung lavish esoteric cosmologies from the parables. Not even the smallest detail was safe from allegorising interpreters. This exegetical excess led second-century writers like Irenaeus and Tertullian to formulate rules for interpreting the parables. Irenaeus' rule is pretty simple (Against Heresies 2.27): stick to the clear kanon of truth, and don't read anything into the parable that would contradict the plain sense of the apostolic faith. For example: don't interpret a parable to mean that there's more than one god, since this would be out of harmony with the faith.
Tertullian is more interesting (On Purity 8-9). He makes the same observation as Irenaeus: interpretations of the parables should be rejected if they "destroy the whole economy of salvation". And he insists that parables should be interpreted in light of the Christian faith: "We do not take take the parables as sources of doctrine, but rather we take doctrine as a norm for interpreting the parables." But he is not concerned only to rule out heterodox interpretations; he also dwells on the characteristics of good interpretation of the parables. He assumes that the good interpreter will take in the whole parable at a single glance, while the heterodox interpreter scrutinises every little detail and loses the wood for the trees. In an age that loved allegory and typology, Tertullian is bracingly straightforward: "Why a 'hundred' sheep? and why, indeed, 'ten' drachmas? and what does that 'broom' stand for? Well, when he wanted to show how pleased God is at the salvation of one sinner, he had to mention some numerical quantity from which one could be described as 'lost'. And in view of the ordinary procedure of a woman who looks for a drachma in the house, he had to supply the assistance of a broom and lamp."
Tertullian thus happily admits that some elements of the parable might remain unexplained. The heterodox interpreters, he says, are driven by a lust for coherence and comprehensiveness. They want every detail to fit. They "work out these parables with perfect consistency." Theirs is a kind of conspiracy-theory theology: a perfect account that explains everything and leaves nothing out. By contrast, good interpreters of the parables should allow for a measure of imperfection and inconsistency. "We make no effort to twist everything so that it fits our own explanation, striving to avoid every discrepancy." An exegetical argument, Tertullian says, "should not be extended beyond the limits of the subject matter with which it is concerned." It is better to have a general sketch of what the parable means than to be lost in a maze of hyper-interpretation. "If needs must be, we prefer to have an incomplete rather than an incorrect understanding of the scripture."
And why does all this matter? Because "bad exegesis is no less serious than bad conduct."