The "belief" is in the God of the Bible, that He is "the same yesterday, today, and forever" and still communicates with humanity. Sometimes He chooses to do this through specific people. A word claiming to be from God needs to be evaluated. The character, reputation, reationship, and track record of the person giving a prophetic word matters in this evauative process. True prophetic people are recognized as such by the community of faith and are accountable to that community. They are not self-proclaimed.