“What does it mean to test the spirits?”
When Paul wrote to the Thessalonians not to despise prophecies but to “test everything; hold fast what is good”, it was around the middle of the first century in one of the first books of the New Testament committed to parchment. The infant church was still in its initial growth spurt, most of the second half of our Bibles was still unwritten, and God spoke frequently through Christian prophets when the Old Testament was insufficient to meet the spiritual needs of gathered believers and provide them with necessary direction from the Head of the Church.
Because prophecy was so frequent, false prophecy was also frequent, so it was necessary to determine when God was really speaking and when he was not.