Paul describes a wonderful activity of the Spirit in verse 16 when he says that ‘the Spirit himself bears witness with our spirit that we are children of God’. We can use three brief words to help us understand this – where, when and how.
The question of where concerns to whom the Spirit is witnessing when Paul says that the Spirit witnesses with our Spirit. Does the apostle mean, for example, that the Holy Spirit tells God in heaven that a person is a believer when that believer speaks to God? Or does the apostle mean that the Holy Spirit informs us on earth that we are believers? Obviously it would be comforting to know that the Holy Spirit remains faithful to his people before God’s throne, but I don’t think that is the type of witnessing that Paul means. Instead, he is describing an ongoing work of the Spirit in the human heart that strengthens and stimulates a believer’s assurance.
When does the Holy Spirit do this? Since Paul is addressing the entire Roman church, it means that he is describing a common Christian experience. By the witness of the Spirit the apostle does not mean a profound occasion when something unusual happened to a Christian. Instead he writes in such a way that he expects his readers to know what he is talking about. The witness of the Spirit is a frequent event; we might even say it can be a continuous activity of the Spirit.
How does the Spirit do this? The answer that has helped me the most with this question is the illustration of a three-legged stool to depict Christian assurance. One leg is the promises of God, a second leg is the deduced changes a person sees in his life as a Christian, and the third leg is the witness of the Spirit.
It is possible for a Christian to have assurance connected to the first two legs – after all, sometimes a two legged stool will stand if the legs are thick enough. Yet in the spiritual life, it is the case that the devil can attack a Christian with regard to those two legs. The devil can tell the Christian that the divine promises are not for him and that the changes he sees in his life are not that great. And the Christian’s assurance begins to waver if that is all that he has.
But it is not all. In addition, the Holy Spirit comes and confirms the divine promises in the believer’s mind so that he sees they are his (the Spirit witnesses with his spirit that they are true and relevant for him) and the Spirit also enables the believer to use self-examination as source of strong assurance by witnessing with him that indeed a great and wonderful change has taken place within his heart and life.