They went out from us, but they didn't belong to us; for if they had belonged to us, they would have continued with us. But they left, that they might be revealed that none of them belong to us.

 Here are the Christian criteria of truth. There is an external test. All teaching must be in accordance with the tradition handed down to us in Scripture and in the Church. There is an internal test. All teaching must undergo the test of the Holy Spirit witnessing within our hearts. 