1 Thessalonians 5:21

Some of the oldest manuscripts insert "But." You ought indeed not to "quench" the manifestations of "the Spirit," nor "despise prophesyings"; "but," at the same time, do not take "all" as genuine which professes to be so; "prove (test) all" such manifestations. The means of testing them existed in the Church, in those who had the "discerning of spirits" (Co1 12:10; Co1 14:29; Jo1 4:1). Another sure test, which we also have, is, to try the professed revelation whether it accords with Scripture, as the noble Bereans did (Isa 8:20; Act 17:11; Gal 1:8-9). This precept negatives the Romish priest's assumption of infallibly laying down the law, without the laity having the right, in the exercise of private judgment, to test it by Scripture. LOCKE says, Those who are for laying aside reason in matters of revelation, resemble one who would put out his eyes in order to use a telescope. hold fast that which is good--Join this clause with the next clause (Th1 5:22), not merely with the sentence preceding. As the result of your "proving all things," and especially all prophesyings, "hold fast (Luk 8:15; Co1 11:2; Heb 2:1) the good, and hold yourselves aloof from every appearance of evil" ("every evil species" [BENGEL and WAHL]). Do not accept even a professedly spirit-inspired communication, if it be at variance with the truth taught you (Th2 2:2).

— Jamieson-Fausset-Brown Commentary (public domain)