Augustine has been in the center of the Evangelical trinitarian controversy on the eternal functional subordination of the Son. Some (Bruce Ware and Wayne Grudem) argue that Augustine taught the eternal functional subordination of the Son in terms of authority. Others (Kevin Giles and Millard J. Erickson) condemn such a view as heretical, arguing that Augustine precluded any type of subordination of the Son in the immanent Trinity. The thesis of this paper is that Augustine really teaches the voluntary ‘relational’ subordination of the Son in eternity. The adjective ‘relational,’ rather than ‘functional,’ is a better term for Augustine’s description of the Son’s eternal dependence upon God. In contrast to Giles and Erickson, I will demonstrate that Augustine seriously took a relational primacy to the Father as the source of the Godhead. As Luigi Gioia nicely points out, therefore, the purpose of revelation through the incarnation and redemption is not to reveal of the three Persons respectively. Instead, the incarnation and redemption in Augustine’s trinitarian theology are to reveal the invisible Father in the incarnate Son in the love of the Holy Spirit. Unlike Ware and Grudem, I would argue, like Lewis Ayres, that the idea of authority would not fit with Augustine’s view of the Son’s eternal relational subordination since it was also the Son himself along with the Father who sent the Son to the world. This paper also presents a considerable theological affinity between Augustine and Karl Barth on the Son’s eternal relational subordination to the Father.