The original Nicene Creed of 325 includes robust theological confessions pertaining to the Father and the Son. However, the Creed gave the Spirit scant attention, merely including “And in the Holy Spirit” concerning the third Person of the Trinity. Over fifty years later at the Council of Constantinople, the Creed enhanced significantly the confession of the Holy Spirit. Rather than merely affirming the Spirit’s existence, the newly revised Creed confesses him as Lord, co-eternal with the Father, and worthy of worship with the Father and the Son. In the decades between both councils, the Church’s pneumatological development came about through a confluence of figures who recognized the glaring weaknesses in the Church’s doctrine of the Spirit both from the so-called “pneumatomachians” who denied the Spirit was God, as well as from the honest questions from earnest believers who didn’t know whether the Spirit was divine or some other entity altogether. Scholars have long noted the special influence of the Cappadocian Fathers, particularly Basil of Caesrea’s “On the Holy Spirit” and Gregory Nazianzus’ “Fifth Theological Oration” that helped create the theological conditions anticipating Constantinople’s re-working of the Nicene Creed. But one major figure who does not always receive due attention is Athanasius. Primarily regarded as the stalwart defender against Arian Christologies of the fourth century, much scholarly exploration has been give to his seminal works “Contra Arianos” and “Contra Gentes – De Incarnatione” given their Christological content. Even though these works proved to be pivotal pivotal for the development of the Church’s understanding of Jesus Christ, Athanasius has some important things to say about the Spirit concerning his relation to the Father and Son, and his work in salvation. In addition to these texts, his lesser-celebrated “Letters to Serapion” convey the maturity of his pneumatology as he answers challenging questions concerning the Spirit’s deity and personhood. Though rightly recognized for his influence on fourth century Christology, he also played a major role in the development of pneumatology. Therefore, in this paper I hope to draw out important themes in Athanasius’ pneumatology as it unfolds chronologically throughout his writings, and demonstrate that he played a vital role in helping the Church develop its pneumatology culminating at Constantinople.