1 Thessalonians 5:21

21. Prove That is, probe, try, test, put to the test. The word is specially used of testing counterfeit coin by ringing, weighing, fire, or touchstone. And so a precept was traditionally ascribed to our Lord, “Be ye skilful testers of coin.”

All Not things, but charisms, or professed supernatural gifts of the Spirit, as well as the doctrines they propound. And this is equivalent to 1 John 4:1: “Believe not every spirit, but try the spirits whether they are of God.” This duty is enjoined upon private Christians, and so, is an assertion of the right of private judgment. It admits of no pope, to impose a spirit or a doctrine upon us without any of our own trying or testing. The tests for a doctrine are pre-eminently Scripture, the consent and harmony of holy men, our own conscience, our own sense of decency and propriety, and our sober common sense. It is possible that even these Thessalonians had one or two written gospels. They had the counsel of those apostles whose spirit they had tried and found not wanting. They were, doubtless, much endowed with the gift of discerning spirits. As, then, they had the tests in their hands, they must not despise or quench indiscriminately, but test, criticise calmly, select wisely, and form a permanent conclusion.

Hold fast… good When the good (spirit or doctrine) was critically and fairly found, they should grasp and hold it fast as a divine acquisition. It is more precious than rubies. It is a coin of the sanctuary that will open the gates of heaven.