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?
If they do, then we know the Spirit of God has moved them to make such a confession. Anyone who opposes Christ's incarnation, and therefore His passion, death, and resurrection are bearing the spirit of antichrist. It is really not difficult to distinguish among professed Christians and Christian teachers, those who are heavenly in their conversation from those who are influenced by the spirit of the world. Those of the truth will hear the truth and those who are not won't. What make some of the truth and others not, only God can say - but those who are of it will always hear it.