- Scripture has such a unity that it never contradicts itself; every part of it bears harmonious and consistent witness to all articles of faith. So we apply the analogy of faith to every teaching or interpretation, comparing it to this harmonious testimony of Scripture.
- The unity of Scripture also extends to the local level, where clauses, sentences, and paragraphs stand in meaningful relation to each other. So we interpret every biblical statement in light of its context; otherwise we would do violence to its true meaning.
- Scripture is not made of identical parts; it is a unity of different but complimentary parts, each serving a particular purpose. One passage establishes this article of faith, another establishes that article. One passage speaks to its purpose very clearly and directly, another passage less so. Subject to context and the analogy of faith, we use the clearest passages that most directly establish an article of faith (sedes doctrinae) to clarify our reading of less clear and direct verses related to the same teaching.
Zwingli & Co. used a wide selection of Bible passages to "prove" that the doctrine of the Sacrament - that we actually eat Christ's body and drink his blood with our mouths - is foreign and even repugnant to Scripture. So when Jesus says, "I am the bread of life; he who comes to Me shall not hunger, and he who believes in Me shall never thirst" (John 6:35), this "proves" that when He speaks soon afterward about eating His flesh and drinking His blood (6:51), it is only a metaphor for "coming" to Him and "believing" in Him. And when Jesus says, "It is the Spirit who gives life; the flesh profits nothing" (6:63), this "proves" that no spiritual benefit (such as forgiveness) can be attached to fleshly eating (such as eating His body). So from the Reformed perspective, Zwingli's party made an ironclad biblical case (which Luther either would not or could not answer), "proving" clearly that the "eating and drinking" in the Sacrament is purely spiritual, and consists of faith.
Luther "would not or could not" answer this because he kept going back to the words "THIS IS MY BODY," which he famously wrote on the table so that he could repeatedly pull back the cloth and reveal them again. In the eyes of Reformed scholars this was a feeble defense by a stubborn, tradition-bound man who refused to yield even to Zwingli's larger amount of biblical ammunition. But in other eyes - eyes conditioned to reading Scripture under profoundly different principles - Luther triumphed. He triumphed on the strength of the sedes doctrinae principle of Bible interpretation. He triumphed by confessing, firmly and clearly, that the key to the Sacrament is what Christ said in His last will and testament (Matthew 25:26-28; Mark 14:22-24; Luke 22:19-20; 1 Corinthians 11:24-25). This is the clearest testimony of God's Word on what is in the Lord's Supper. In these words, which have undeniable and immediate application to the Sacrament, Jesus declared: "This [bread] is my body...This [cup] is my blood...for the forgiveness of sins."
The passages Zwingli adduced may or may not be remotely related to the Lord's Supper. So the proper order is to interpret these John 6 quotes by reference to Jesus' words of institution, rather than vice versa. After the analogy of faith (which Zwingli challenged) and the context (which makes no reference to the Sacrament), the most decisive filter or lens for evaluating a Bible verse's relevance to doctrine is the sedes doctrinae.
Applying these principles to John 6:35 and John 6:63 reveals that these verses have no bearing on the doctrine of the Lord's Supper. They are not teaching about the same thing. They do not contradict, correct, or obliterate Jesus' plain, literal teaching, at the most serious moment of his earthly life, that what He gives us to eat in the Sacrament is His body for our forgiveness. By some vague association of ideas one might draw an analogy between the Lord's Supper and what Jesus teaches in John 6; in fact, I dare say it is impossible for a biblical teacher not to do so. But analogy of faith, context, and sedes doctrinae dispose of any attempt to turn Jesus' words in John 6 against His all-important words, "THIS IS MY BODY." Whoever would reason differently, quote as many Scriptures as they may, must be driven against these words as the wind drives a ship against a rock.
"Scripture interprets Scripture" is a distinctively Lutheran hermeneutic. I suppose the phrase may drop off the lips of other Protestant teachers; but if they took it seriously, they would teach differently. It does not mean the Bible is a democracy of equally-weighted words that can be brought into contention with each other, so that the side supported by the most words wins. Rather, the sedes doctrinae principle means that Bible passages relating to a doctrine can be arranged in a hierarchy, with those that most plainly and directly establish the doctrine holding authority over the rest.