The ability to prophesy was one of the most important gifts in the New Testament (see also 1 Corinthians 12:28; Ephesians 4:11). Prophets in the book of Acts often predicted the future (see Acts 11:28; 21:10–12). But their main role was to share God's message with Christians (1 Corinthians 12:3, 24–25, 29–30; see also 1 Corinthians 14:1–40).
in proportion to his faith: The word proportion (Greek analogia) comes from mathematics and logic. It refers to a proper relationship between things, amounts, or ideas.
Paul uses this phrase to remind prophets to speak in a way that matches faith. This may refer to the amount of faith God has given to the prophet. It may also refer to the shared Christian faith held by all believers (see study note on Romans 12:3).
This passage later became the basis for the theological phrase the analogy of faith. This phrase means that any one part of Scripture should be understood in a way that agrees with the teaching of all of Scripture.