The ‘analogy of faith’ (an expression borrowed from Rom 12:6, but not reflecting Paul’s sense there) as a principle of interpretation ‘connotes that an obscure text or passage may be illuminated by other texts of Scripture whose meaning is clear.’ (Evangelical Dictionary of Theology)

There are two degrees of the analogy of faith

1.  Positive Analogy.  This consists of those teachings of the Bible that are so clearly, positively and frequently stated, that there can be no real doubt of their meaning and value.  Such truths include the existence of God; his holiness and righteousness; his providential rule, the heinousness of sin; redeeming grace as revealed in Jesus Christ; and a future life and retribution.

2.  General Analogy.  This does not rest on explicit statements of Scripture, but on the general tenor and drift of its teachings.  Such truths include the hostility of Scripture to formalism in worship, and the desirability of spiritual worship.

‘As a connoisseur, in judging a masterpiece of painting, fixes his attention, first of all, on the central object of interest, and considers the details in the relation to this; so the interpreter must study the particular teachings of the Bible in the light of its fundamental truths.’

The value and authority of the analogy of faith will vary according to a number of factors:-

1.  The number of passages that contain the same doctrine
2.  The unanimity or correspondence of the different passages
3.  The clearness of the passage
4.  The distribution of the passages

When employing the analogy of faith, the interpreter should bear in mind the following:-

1.  A doctrine that is clearly supported by the analogy of faith cannot be contradicted by a contrary and obscure passage.  Think of 1 Jn 3:6, and the general teaching of the Bible that believers do sin.
2.  In cases where the analogy of Scripture leads to the establishment of two doctrines that appear contradictory, both should be accepted as Scriptural in the confident believe that they resolve themselves into a higher unity.  Example: the doctrines of total depravity and human responsibility.

J. I. Packer explains the Puritan’s use of the analogy of faith as follows:

‘Since Scripture is the unified expression of a single divine mind, it follows that “the infallible rule of interpretation of Scripture is the Scripture itself, and therefore, when there is a question about the true and full sense of any Scripture … it must be searched and known by other places that speak more clearly” (Westminster Confession).  Two principles derive from this.

1. ‘What is obscure must be interpreted by the light of what is plain. “The rule in this case,” says Owen, “is that we affix no sense unto any obscure or difficult passage of Scripture but what is … consonant unto other expressions and plain testimonies. For men to raise peculiar senses from such places, not confirmed elsewhere, is a dangerous curiosity.”
2. ‘Peripheral ambiguities must be interpreted in harmony with fundamental certainties. No exposition of any text, therefore, is right which does not “agree with the principles of Religion, the points of Catechism set down in the Creed, the Lord’s Prayer, the Ten Commandments, and the doctrine of Sacraments.”

See Berkhof, Principles of Biblical Interpretation, pp163-166; Packer, Among God’s Giants, p134f.