It is often assumed that the Holy Spirit's witness to a believer is not very helpful in a study of apologetics. After all, this testimony is given only to Christians and it is not verified or falsified by evidences. So does it follow that this witness is no more than a subjective conviction?
In the few NT passages that address this subject, we are told that, at a minimum, the witness of the Holy Spirit is a personal word to believers that they are children of God (Rom 8:15-17). The Holy Spirit testifies to believers as family members (Gl 4:6-7). So the believer will experience the presence of the Holy Spirit (Jn 14:16-17). This is one way to know that we are truly believers (1 Jn 3:24; 4:13).
Since the unbeliever cannot understand things pertaining to salvation (Jn 14:17; 1 Cor 2:14), one might questions the value of the Holy Spirit's witness in an apologetic context. But this seems to assume that dealing with unbelievers is the only purpose for defending the faith. Apologetics may have even more value in strengthening the faith of believers through a variety of avenues.
Since the chief purpose of the Holy Spirit's witness is to provide personal assurance of the believer's salvation, the resulting confidence can play a valuable role in convincing believers of their own relationship with the Lord. This might provide assistance, for example, in dealing with religious doubt.
Moreover, the witness of the Holy Spirit provides indirect confirmation of the truth of the Christian gospel. After all, if we are the children of God and joint heirs with Jesus Christ as we suffer and are glorified with Him (Rm 8:17), then it would follow that God's gospel path--the basis of this assurance--is likewise true.
So when people become Christians and experience the Holy Spirit's presence, it ought not surprise them, since this is precisely what Scripture teaches! It should be normal fare for the believer. Briefly stated, the study of apologetics indicates that Christianity is true; the witness of the Holy Spirit performs the related function of identifying those who are members of the faith.