The theology of Paul Tillich has been criticized for its "apparent" lack of reference to Scripture. Tillich's work, while entrenched in the Christian theological and philosophical tradition, has been scrutinized often. If there are no explicit references in his Systematic Theology to Scripture, then to what extent can he be labeled a Christian theologian? Is this a valid question, or should he be considered only in terms of philosophical theology and of world religions? John Charles Cooper has researched this topic for thirty years, beginning as a student of Tillich's in Chicago. Cooper clearly demonstrates that using Paul as an example, the apostle's understanding of the Holy Spirit and the Spirit of Christ are determinative of Tillich's doctrine of Spiritual Presence.