Yet the Scriptures themselves teach us a healthy kind of caution and skepticism. Did you know there is actually a command not to believe?
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 (1 John 4:1).
Sincerity is a good thing, right? Not if it is sincerely wrong. Jews who reject Jesus “have a zeal for God, but not according to knowledge” (Romans 10:2 cf. Acts 21:20). In their zeal and ignorance, they crucified Christ (Luke 23:34; Acts 3:17). Saul (Paul) was sincere when he was killing Christians (Acts 22:3-4; 23:1; 26:9). Jesus told His apostles, “The hour is coming when whoever kills you will think he is offering service to God” (John 16:2).
Actually, sincerity can pose the greater danger. The person who knowingly deceives others can be quite convincing. But even more convincing is the person who passionately believes he speaks for God. Such sincerity may well be part of the “sheep’s clothing” that most effectively covers up the wolf of falsehood (Matthew 7:15). Think about the amazing scene foreseen by Matthew 7:21-23. Jesus will sit as Judge in all His glory (cf. Matthew 25:31-32). He already knows every heart (Acts 1:24). Yet, even in that awesome setting, many false prophets will feel so sure of their own rightness that they will actually dispute with Jesus face to face (Matthew 7:21-23). Apparently they will argue because they have been deluded so completely. If they will be so sure of themselves there, how sincere are they here, among us? Right now, how many people does their sheer boldness lead astray? Such sincerity, however, will not prevent the final sentence, “I never knew you; depart from Me, you workers of lawlessness.”