“The Spirit himself bears witness with our spirit that we are children of God . . .” Romans 8:16
This “witness” which leads to assurance that we are in God’s family is not a direct or immediate feeling or sense that we are God’s children—some inner confidence that may be entirely misplaced by being based on nothing but the notion itself. The believer will not say merely, “I know that I know that I know.” In fact, having confidence in a feeling or inner sense that something is so can be the purest form of deception.
As an illustration, I once knew a woman who believed she was in a romantic relationship with a radio personality that she listened to late at night. She literally flew to the city where this person recorded his program to be with him. But from his perspective, he not only did not have romantic feelings for this woman, but had never even met her. She was sure based on nothing but an inner feeling.
The “witness of the Spirit” rather is the testimony that the Spirit produces in your life by the changes he causes and points to. In context such evidence consists of such things as “putting to death the deeds of the body,” being “led by the Spirit” (that is, led morally into obedience as a pattern of life), and the consistent desire to communicate with your Father with affection (see 8:14-17).