Jamieson, Fausset and Brown on 1 Thessalonians 5:19

19. Quench not--the Spirit being a holy fire: "where the Spirit is, He burns" [Bengel] (Mt 3:11; Ac 2:3; 7:51). Do not throw cold water on those who, under extraordinary inspiration of the Spirit, stand up to speak with tongues, or reveal mysteries, or pray in the congregation. The enthusiastic exhibitions of some (perhaps as to the nearness of Christ's coming, exaggerating Paul's statement, 2Th 2:2, By spirit), led others (probably the presiding ministers, who had not always been treated with due respect by enthusiastic novices, 1Th 5:12), from dread of enthusiasm, to discourage the free utterances of those really inspired, in the Church assembly. On the other hand, the caution (1Th 5:21) was needed, not to receive "all" pretended revelations as divine, without "proving" them.