What does 1 Thessalonians 5:21 mean?

This verse follows Paul's instruction for the Thessalonians not to "despise" prophecies. This does not mean accepting any and every claim of prophetic speech, of course. Here, Paul helps his readers know how to respond to prophecy. He tells them to examine every prophecy to determine whether it is valid. If it passes examination, in the light of what the Scriptures teach (Isaiah 8:20), the believers should welcome it. The apostle John affirms Paul's admonition regarding prophecy. In 1 John 4:1 he writes: "Beloved, do not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world." In a broader, sense, this is a useful tool for believers to apply to everything in their spiritual lives: a cautious, Scripture-tested approach.

Ephesians 4:11–12 lists "prophets" among those whom God gave to the church to help her develop spiritually. Now that God's Word has been delivered, the need for this kind of ministry has passed. The equivalent to "prophesying" today can be described as the act of forth-telling, not fore-telling. This is carried out by the forth-telling of God's written Word by teachers and preachers. Believers today should test messages that teachers and preachers claim to be from God. The test is not whether a preacher looks good or has a pleasing personality; it is whether his message aligns with the Word of God.