The principle that Scripture interprets Scripture — also called the analogy of faith (analogia fidei) — is the Reformed doctrine that the infallible rule of interpretation is Scripture itself. When there is a question about the true and full sense of any passage (which is not manifold, but one), it must be searched and known by other places that speak more clearly. ^[raw/en/wcf-ch01-s09.md]
2 Peter 1:20-21 establishes the principle: "no prophecy of the scripture is of any private interpretation." The Greek epiluseos (interpretation) means loosing, unbinding — a knot untied. Peter is not forbidding personal reading but insisting that the meaning of Scripture is not the product of isolated private judgment.
1 Corinthians 2:13 gives the method: "comparing spiritual things with spiritual" (sunkrinontes — combining, fitting together, interpreting one by another). Paul tells us the proper method is not to isolate but to collate — bringing texts into conversation with one another.
Our Lord Himself demonstrated this on the road to Emmaus: "beginning at Moses and all the prophets, he expounded unto them in all the scriptures the things concerning himself" (Luke 24:27). Christ did not take a single prophecy and impose a meaning; He traversed the whole field of Scripture.
Isaiah 34:16 declares: "Seek ye out of the book of the LORD... none shall want her mate" — every passage has its interpreting partner within the book itself. ^[raw/en/wcf-ch01-s09.md]
The Confession affirms that the true and full sense of any Scripture "is not manifold, but one." This repudiates the medieval fourfold sense of Scripture (literal, allegorical, tropological, anagogical) which turned the Bible into a "wax nose." Following the Reformers' grammatical-historical method, the Divines insisted every passage has one true meaning: what the human author intended under the superintending inspiration of the divine Author. ^[raw/en/wcf-ch01-s09.md]
This does not deny typology or multiple applications. As Warfield explains, the same passage may have historical application, typological depth, and moral application — all aspects of the one true sense.
John Owen writes: "The only infallible rule of interpretation is, that the Scripture itself be admitted to declare its own meaning, by comparing one place with another." He distinguishes the analogy of faith (the constant and invariable sense of Scripture in all necessary truths) from the scope of a passage (the principal design of the Holy Ghost in that portion).
Thomas Watson illustrates: "As the diamond only cuts the diamond, so the Scripture only is to interpret the Scripture." Francis Turretin explains that the analogy of faith functions both positively (directing us to interpret obscure passages by clear ones) and negatively (forbidding any interpretation that contradicts clear teaching elsewhere).
The principle guards against building doctrine on isolated proof texts — the source of every heresy. It requires that the whole Bible be known and brought to bear on every part. This is why thorough knowledge of Scripture is essential for every believer.
This doctrine connects to clarity-of-scripture (the clear passages interpret the obscure) and to authority-of-scripture (the Spirit speaking in Scripture is its own best interpreter). It also undergirds the sufficiency-of-scripture: if Scripture can interpret itself, no external authority (magisterium, private revelations) is needed.