Adam Clarke on 1 Thessalonians 5:21

Prove all things - Whatever ye hear in these prophesyings or preachings, examine by the words of Christ, and by the doctrines which, from time to time, we have delivered unto you in our preaching and writings. Try the spirits - the different teachers, by the word of God.

Hold fast that which is good - Whatever in these prophesyings has a tendency to increase your faith, love, holiness, and usefulness, that receive and hold fast. There were prophets or teachers even at that time who professed to be of God, and yet were not.