This verse is not a license to investigate all kinds of things, but it gives an indication of how to deal with the prophecies written about in the preceding verse. Not every prophecy is necessarily good. It must be tested. The Teachings of the Twelve Apostles, an early church writing, says that not everyone who speaks in the Spirit is a prophet. This is only the case if he follows the Lord’s way of life. From their way of life one can know the false and the true prophet. The prophecy must fit the whole of Scripture (see Acts 17:11). If so, it should be gratefully accepted.
21 but test everything; hold fast what is good.