Prove all things; hold fast that which is good.
B Delta G f g, Vulgate, insert, 'but.' 'Aleph (') A C omit it. You ought indeed not to "quench the Spirit" nor "despise prophesyings;" 'but,' at the same time, do not take "all" as genuine which professes to be so; 'prove (test) them all.' Means of testing existed in the Church: some had the "discerning of spirits" (1 Corinthians 12:10; 1 Corinthians 14:29; 1 John 4:1). Another test, which we also have, is to try the professed revelation whether it accord with Scripture, as the noble Bereans did (Isaiah 8:20; Acts 17:11; Galatians 1:8-9). This negatives Rome's assumption of infallibly laying down the law: the laity have the right of private judgment, and are bound to exercise it in testing every human teaching by Scripture. Locke, 'Those who are for laying aside reason in matters of revelation resemble one who should put out his eyes to use a telescope.'
Hold fast ... good. Join this with the next clause (1 Thessalonians 5:22), not merely with the preceding. As the result of 'proving all things,' especially prophesyings, 'hold fast (Luke 8:15; 1 Corinthians 11:2; Hebrews 2:1) the good: hold yourselves aloof from every species of evil.' [Ellicott: as the antithesis to "that which is good," and the lexical meaning of eidous, 'form,' favour. But see below]. Accept not even a professedly spirit-inspired communication, if at variance with the truth taught you (2 Thessalonians 2:2). Tittmann supports, 'from every evil appearance.' The context refers not to standing aloof from every evil appearance IN OURSELVES but IN OTHERS; for instance, pretended spirit-inspired prophesyings. The Christian often should not abstain from what has the "appearance" of evil, when really good. Jesus healed on the Sabbath, and ate with publicans-acts which wore the appearance of evil, but which were really good. The context favours this sense: However specious be such pretended prophets and their "prophes yings," stand aloof from every such appearance when it is evil. [Luke 9:29 justifies this sense of eidos (G1491) lexically].