(1) How can you spot a false prophet? The Bible offers several complementary criteria. For instance, in Deuteronomy 18:22 we are told that if an ostensible prophet predicts something and that thing does not take place, the prophet is false. Of course, that criterion does not help very much if what the prophet has predicted is far into the future. Moreover, here in Deuteronomy 13 we are warned that the inverse does not prove the prophet is trustworthy. If what the ostensible prophet predicts takes place, or if he manages to perform some sort of miraculous sign or wonder, another criterion must be brought to bear. Is this prophet’s message enticing people to worship some god other than the Lord who brought the people out of Egypt?
What this criterion presupposes is a thorough grasp of antecedent revelation. You have to know what God has revealed about himself before you can determine whether or not the prophet is leading you to a false god. For the false god may still be given the biblical names of God (as in, say, Mormonism, or the christology of Jehovah’s Witnesses). John’s first epistle offers this same criterion: if what an ostensible prophet (1 John 4:1-6) teaches cannot be squared with what the believers have heard “from the beginning”(1 John 2:7; 2 John 9), it is not of God (so also Paul in Gal. 1:8-9).
(2) Why are false prophets dangerous? Apart from the obvious reason, viz. that they teach false doctrine that leads people astray from the living God and therefore ultimately attracts his judgment, there are two reasons. First, their very description — “false prophet”– discloses the core problem. They profess to speak the word of God, and this can be seductive. If they came along and said, “Let us sin disgustingly,” most would not be attracted. The seduction of false prophecy is its ostensible spirituality and truthfulness. Second, although false prophets may enter a community from outside (e.g., Acts 20:29 — and if it is the “right” outside, this makes them very attractive), they may arise from within the community (e.g., Acts 20:30), as here — for example, a family member (Deut. 13:6). I know of more than one Christian institution that went bad doctrinally because of nepotism.
(3) What should be done about them? Three things. First, recognize that these testing events do not escape the bounds of God’s sovereignty. Allegiance is all the more called for (Deut. 13:3-4). Second, learn the truth, learn it well, or you will always lack discernment. Third, purge the community of false prophets (a process that takes a different form under the new covenant: e.g., 2 Cor. 10 — 13; 1 John 4:1-6), or they will gradually win credence and do enormous damage.
Source: For The Love of God by Don Carson (Following Robert Murray Mccheyne reading plan)