While it's true we have false prophets run amok today, who are more false teachers than those claiming to actually be prophets, John's advice in these verses was to them who were surrounded with divisions and threats to the faith. What John was really warning them about was people who claimed to be speaking for God. The claim at hand was that Christ was not a man of flesh and blood. John was there to tell them otherwise. He says here that every spirit that admits Jesus Christ came in the flesh is of God. He doesn't say this confession makes them a Christian or a saved individual. This is a starting point for us as believers when talking with people - do they believe that Jesus came in the flesh?