‘Do not quench the Spirit, do not despise prophesyings, prove all things, hold fast that which is good, abstain from every appearance (or ‘form') of evil.'
The word for quench is often used of the quenching of a fire, but the word then means ‘stifle, suppress'. We quench the Spirit when we choose to sin, we quench the Spirit when we allow other things to take over our thoughts that should not, we quench the Spirit when we have a harsh attitude, we quench the Spirit when we are not willing to go along with God. But the main thought here would seem to be quenching the Spirit by being over-critical and by unwillingness to hear those who proclaim the truth, by formalism and possibly by being unwilling to discern the Spirit at work through unexpected sources.
Following on Paul's previous threefold injunctions it may be that we should divide this as, ‘Do not quench the Spirit, do not despise prophesyings --- prove all things, hold fast that which is good --- abstain from every appearance of evil'.
‘Prophesying' refers to bringing a message ‘from God'. It has to be proved and tested (compare also 1 Corinthians 14:29). There is no thought of just accepting what is said, nor would it be open to just anyone. ‘Prophets' would be acknowledged as having the ‘gift of prophecy' (1 Corinthians 12:29; Ephesians 4:11). They were formally recognised (Acts 13:1; Ephesians 2:20; Ephesians 3:5). Their message would be tested against the Scriptures. But there would be a number of them in each church gathering (1 Corinthians 14:29-31). Their main ministry was exhortation (Acts 15:32). At least one, Agabus, had a special gift for interpreting the near future (Acts 11:28; Acts 21:10), but note that neither event was a detailed prediction, and both referred to something discernible by clear insight based on signs already present. The point was that the insight was confirmed by God. He was not a fortune teller.
It would seem that a feeling had arisen against such prophesying among some, possibly because the message being given was unwelcome. And this may have led to attempts to limit the number of prophets. Paul warns that while the prophets have to be tested, their message should not be despised, and their gifts should be recognised.
‘Prove all things, hold fast that which is good.' The church, however, must be constantly on the guard against error. Everything must be tested, both by other prophets (1 Corinthians 14:29) and ultimately by the Scriptures. This not only refers to prophesying, but to all things pertaining to the church. Then what is good must be held fast, and the rest rejected. Christians were not to be undiscerning.
‘Abstain from every appearance (or ‘form') of evil.' The Christian is to avoid evil in all its forms, whether in false teaching, in false dealings or in false living. They must avoid that which even gives an appearance of evil. It is the opposite of holding fast what is good. An example of abstaining from ‘the appearance of evil', that which may cause someone else to stumble, is found in Romans 14:15-16; Romans 14:21.