What significant topic was much of St. Gregory of Nazianzus's writings focused on?

St. Gregory of Nazianzus, one of the Cappadocian Fathers, is particularly noted for his contributions to the theological understanding of the Holy Spirit. His writings extensively address the nature, divinity, and role of the Holy Spirit within the context of the Trinity, emphasizing the Spirit's co-equality with the Father and the Son. This was especially significant during a time when the Arian controversy questioned the full divinity of the Son and, by extension, the Holy Spirit.

Gregory's theological reflections helped to counter the views of those who minimized the Holy Spirit's role in salvation and the life of the Church, contributing to the formulation of key doctrines that would later be clarified and affirmed in the Nicene Creed. His contributions laid important groundwork for the eventual recognition of the Holy Spirit as a fully divine person, pivotal to the Christian understanding of God as a Trinity of Father, Son, and Holy Spirit.

This focus on the Holy Spirit is why the correct answer pertains to Gregory's primary theological emphasis, highlighting his role as a critical figure in the development of early Christian doctrine related to the Trinity.