6. We—true teachers of Christ: in contrast to them.
are of God—and therefore speak of God: in contrast to "speak they of the world," 1Jo 4:5.
knoweth God—as his Father, being a child "of God" (1Jo 2:13, 14).
heareth us—Compare Joh 18:37, "Every one that is of the truth, heareth My voice."
Hereby—(1Jo 4:2-6); by their confessing, or not confessing, Jesus; by the kind of reception given them respectively by those who know God, and by those who are of the world and not of God.
spirit of truth—the Spirit which comes from God and teaches truth.
spirit of error—the spirit which comes from Satan and seduces into error.