Prove all things; hold fast that which is good.

People's New Testament

5:21 Prove all things. What was spoken they must put to the test, they must hold fast to. There were those who could discern spirits (1Co 12:10). There were also rules for testing (1Jo 4:1-8).

Wesley's Notes

5:21 Meantime, prove all things - Which any preacher recommends. (He speaks of practice, not of doctrines.) Try every advice by the touchstone of scripture, and hold fast that which is good - Zealously, resolutely, diligently practise it, in spite of all opposition.

Jamieson-Fausset-Brown Bible Commentary

21, 22. 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" (1Co 12:10; 14:29; 1Jo 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; Ac 17:11; Ga 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 (1Th 5:22), not merely with the sentence preceding. As the result of your "proving all things," and especially all prophesyings, "hold fast (Lu 8:15; 1Co 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 (2Th 2:2).

Matthew Henry's Concise Commentary

5:16-22 We are to rejoice in creature-comforts, as if we rejoiced not, and must not expect to live many years, and rejoice in them all; but if we do rejoice in God, we may do that evermore. A truly religious life is a life of constant joy. And we should rejoice more, if we prayed more. Prayer will help forward all lawful business, and every good work. If we pray without ceasing, we shall not want matter for thanksgiving in every thing. We shall see cause to give thanks for sparing and preventing, for common and uncommon, past and present, temporal and spiritual mercies. Not only for prosperous and pleasing, but also for afflicting providences, for chastisements and corrections; for God designs all for our good, though we at present see not how they tend to it. Quench not the Spirit. Christians are said to be baptized with the Holy Ghost and with fire. He worketh as fire, by enlightening, enlivening, and purifying the souls of men. As fire is put out by taking away fuel, and as it is quenched by pouring water, or putting a great deal of earth upon it; so we must be careful not to quench the Holy Spirit, by indulging carnal lusts and affections, minding only earthly things. Believers often hinder their growth in grace, by not giving themselves up to the spiritual affections raised in their hearts by the Holy Spirit. By prophesyings, here understand the preaching of the word, the interpreting and applying the Scriptures. We must not despise preaching, though it is plain, and we are told no more than what we knew before. We must search the Scriptures. And proving all things must be to hold fast that which is good. We should abstain from sin, and whatever looks like sin, leads to it, and borders upon it. He who is not shy of the appearances of sin, who shuns not the occasions of it, and who avoids not the temptations and approaches to it, will not long keep from doing sin.