This verse comes at the very end of the first letter of Paul to the church in Thessaloniki. It is part of a series of instructions that Paul gives to that church, which, if they keep, will result in their ‘whole spirit, soul, and body being preserved blameless at the coming of our Lord Jesus Christ’ (vs 23).
So let’s look in detail at our verse today. We have to test all things. The Greek word for test is ‘dokimazo’, which means to test, examine, scrutinize, or to recognize as something genuine after examination. It is how people would test gold coins, to see if they are the real deal. So what is it that we have to test?
All things! The word for all things is pas, which means indeed all, or all things. So we have to carefully examine everything. The prior verse mentioned that we should not ignore or despise prophecies, and many of the Greek manuscripts have a ‘de’ (but) in verse 21. This would render the meaning that we should not ignore prophecies but carefully examine them all. Some of the oldest manuscripts do not have this 'de', so we can play it save and keep our scope broad.
In our day and age, this means that we have to carefully examine everything that people are trying to teach us about God, Christianity, and truth, whether they call it prophesy or not. A popular view is that all roads lead to the top of the mountain, but let's examine the logic in that. Googling 'are all religions true' gives this statement as its first result from debate.org: "Since different religions say different things they cannot all be true. It is possible from that perspective that they could all be false, but there can be no more than one truth. The Muslims and Jews say Jesus is not God just a prophet, the Christians say he is God. They can't all be right."
This logic is impossible to contradict and thus it is true. There are many contradicting views among and within religions. So we have to examine everything, carefully. But how exactly do we do that? The church in Berea gives us a good example: “they examined the scriptures daily to see if these things were true” (Acts 17:11). So when someone tries to teach you something, you have to be able to go to the scriptures to test if what they say is true.
Now, what do we do once we have examined it? We hold fast what is good. The Greek for holding fast is ‘katecho’, which comes from ‘kata’ (down from), and ‘echo’ (to hold, to have). So it literally means to have down, and it is most often translated as to hold fast. The word for good is ‘kalos’ and it simply means good.
So, the things that we have to hold on to are those that we concluded to be good after careful testing. Not everything you hear is true. Some things are true and good and some things are not good. The responsibility to discern between the two is yours. Not your pastor’s, not some good meaning Christian books or blogs (including this one), but yours and yours alone.
If you do not know how to discern between what is good and not good, you are in trouble. And there is only one-way out: learn how to study your bible. Godspeed!