Both the Catholic theologian David Coffey and the late Protestant theologian Colin E. Gunton treat Augustine's Trinitarian theology in their work. One major common point of their treatment is the theory of the Holy Spirit as the mutual-love between the Father and the Son which Augustine developed. I look upon the two theologians' treatment of Augustine's theory and claim that the difference in their interpretation is shaped by their use and balance of the four sources of theology: scripture, tradition, reason, and experience. I support my claim through four main chapters organized by the four sources. The analysis shows the theologians' different interpretations involving use of scripture, understanding of the 'immanent' and 'economic' trinity and weight on religious experience and the Holy Spirit.