1 John 4:6

We are of God: he that knoweth God heareth us; he that is not of God heareth us not. By this we know the spirit of truth, and the spirit of error.

We are of God ... us ... The apostle's high claim in this is that of "speaking for God in Christ," as one of the plenary representatives of the Son of God on earth and as one of the eyewitnesses of that full gospel which he declared, including his personal and first hand knowledge of the death, burial and resurrection of Jesus Christ. The blunt point of this verse is: that if the false teachers do not agree with the apostles of Christ, they are liars. Everything that was ever advocated in the name of Christianity must pass this test. As Roberts expressed it:

Notice that John sharpens the antithesis, the "us" (the apostolic teachers) and the "them," (the circle of the false teachers). They are two mutually exclusive groups with no neutral ground.[22]

No private teacher could afford to say, as John said here that, "Whoever knows God agrees with me; and only those who are not of God disagree with me."[23] But as regards the holy apostles of Jesus Christ, this is the simple truth. In today's circumstances, this means that those who are of God and those who are not of God are revealed, absolutely, by whether or not their teaching agrees with the New Testament.

By this we know the spirit of truth and the spirit of error ... This is a fourth test of the false teachers, to be considered along with the three tests mentioned in the preceding verse. There is nothing exhaustive about this list of tests; John's extensive teaching on the tests of determining genuine Christianity reveal others.

[22] J. W. Roberts, The Letters of John (Austin, Texas: The R. B. Sweet Company, 1968), p. 109.

[23] John R. W. Stott, op. cit., p. 158.