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Proportion of faith
Paul says here that each one was to exercise his gift of prophecy, i.e., of teaching, “according to the proportion of faith.” The meaning is, that the utterances of the “prophet” were not to fluctuate according to his own impulses or independent thoughts, but were to be adjusted to the truth revealed to him as a beliver, i.e., were to be in accordance with it.
In post-Reformation times this phrase was used as meaning that all Scripture was to be interpreted with reference to all other Scripture, i.e., that no words or expressions were to be isolated or interpreted in a way contrary to its general teaching. This was also called the “analogy of faith.”