First John 4:1-3 offers a Christological criterion for distinguishing between true and false prophecy; verses 4-6 add an ecclesiological one.
You, dear children, are from God and have overcome them, because the one who is in you is greater than the one who is in the world. They are from the world and therefore speak from the viewpoint of the world, and the world listens to them. We are from God, and whoever knows God listens to us; but whoever is not from God does not listen to us. This is how we recognize the Spirit of truth and the spirit of falsehood.
Notice, first of all, the “you versus them” language in this passage. John is writing to churches in or around Ephesus that have recently experienced a church split over doctrinal and ethical issues (Regarding the church split, see 1 John 2:19; regarding the doctrinal and ethical issues, see 2:22-23 and 1:8-10). Unfortunately, the members on the “wrong side” of the split—the one that taught false doctrine and bad ethics—continued to consider themselves Christians of some sort. Indeed, they seem to have grounded their ersatz Christianity in spiritual revelations. This is what prompted John to offer Christological and ecclesiological criteria for distinguishing between true and false prophecies.
Second, notice the “from” language. According to John, “you versus them” is not an arbitrary description, the result of a power play between sides in a first-century church split. Rather, the “you versus them” distinction bespeaks great spiritual realities. “You” are “from God.” “They” are “from the world.” “You” have been saved; “they” are still caught in sin. “You” are authentic Christians; “they” are counterfeit ones.
Third, therefore, notice that which group you belong to serves as a rational criterion for distinguishing true and false prophecy. If “you” belong to God and “they” belong to the world, then “you” can speak for God, but “they” cannot. This is an ecclesiological criterion: what church you belong to matters. Or as John says, “This is how we recognize the Spirit of truth and the spirit of falsehood.”
Now, if you are at all like me, you might be a little skeptical of this argumentative strategy. To take a trivial example, suppose a church split over whether the organ should be played in the worship service. And further suppose that both sides of the split had alleged prophecies to back up their side. “Thus saith the Lord: Organ!” Or, “Thus saith the Lord: No organ!” To say that you ought to take my side because true churches always have organs (or never have them) would beg the question under consideration. Right?
But this is not the argumentative strategy John takes. He grounds his teaching about Jesus “in the flesh,” that is, in the historical events of Jesus’ ministry to which he and others were eyewitnesses (1 John 1:1-3). The secessionists, on the other hand, advocated spiritual revelations about Jesus that contradicted eyewitness testimony of Jesus. Eyewitness testimony is more determinative of truth than later, supposedly spiritual revelations. Consequently, you have a greater chance of getting the truth about Jesus and the authentic Christian lifestyle if you join the eyewitness community than if you join the spiritual revelation community.
Whose side you join, in other words, helps you draw closer to Jesus or pulls you farther away from him.