There is a reason why works of Systematic Theology discuss “Theology proper” and “Christology” before they bring up soteriology. In the Scriptures, some concepts must be understood in the context of other concepts, before other concepts can be brought in.
In what follows, Billings discusses a topic of Christology – bringing up the topic of precisely how “the Word became flesh”. Reformed Christology relies on the definition of the Council of Chalcedon (451 AD), which according to Robert Reymond “addressed every problem that had plagued the church [up to that point] with regard to the person of Christ” (Reymond, A New Systematic Theology of the Christian Faith, pg 608). In addition, he says, “In my opinion, as an apologetical, ecumenical, and clarifying statement regarding the person of Christ, the Definition of Chalcedon remains unsurpassed. No other human creed has ever been written that captures as well as it does the exact balance of Scripture and permits all that the Scripture says about God the Son incarnate to be given their just due”.
That’s not the end of the story by any means. But moving then to the concept of “Union with Christ” (a topic of “soteriology” rather than “Christology”), the following selection discussing how Augustine, relying upon the pre-Chalcedonian Christology of his day (he wrote vs Pelagius approximately in the years 412-415) ties together both the Christology and soteriology to come up with his understanding of Sovereign grace:
