Jesus is not warning about all types of false prophets in this passage. He is warning about those who profess to be Christian ministers(They are in Sheep's clothing).There are at least five things in the context of this passage that describes this kind of false prophet that makes them so hard to identify. 1. They seem and sound doctrinally sound. 2. They zealously and unashamedly confess Jesus to be their Lord. 3. They preach in the name of Jesus. 4. They are often very gifted and powerful preachers. 5. They are often decieved as to their own spiritual state and true identity.