Author Note: The following is a section of a talk I gave on the obedience of the eternal Son. The whole paper will be published later this year.
God is in and of himself perfect. He has nothing to become, increase, or improve. Suffering no lack, or need, nor having any potential to diminish or end, God is complete in his own plenitude of perfection. God needs nothing other than himself and as such, God is simple—incomposite and indivisible: he is not made of parts nor can he be divided into parts.
God is who he is as Father, Son, and Spirit. Since God is simple, the persons are not parts of the God nor are they additional to the divine essence. Rather, they are the divine essence itself subsisting in a particular manner by way of relations of origin. Hence, God’s life is the very eternal relations of the divine persons. Since the persons are identical in essence, the only way one can distinguish them is by their immanent relations, which indicate their personal properties.
A personal property belongs to one person and not the other two and is on account of the eternal relations of origin (or divine processions)—relations that tell us this one eternally proceeds from that one. Such relations and properties are made manifest not only in the divine missions, but in the scriptural names such as Father, Son, and Spirit. The Father’s personal property is paternity; the Son’s personal property is filiation/generation; the Spirit’s property is procession. Such names and properties help us understand the intratrinitarian relations of origin. For example, since one is Father and the other is Son, we know the Father eternally begets the Son, and thus the Son is from the Father through eternal generation/begetting.
The eternal generation of the Son by the Father refers to, in Turretin’s words, “a communication of essence from the Father (by which the Son possesses indivisibly the same essence with him . . . ).” In John Webster’s words, “Eternal generation is the personal and eternal act of God the Father whereby he is the origin of the personal subsistence of God the Son, so communicating to the Son the one undivided divine essence.” To speak of the Son in light of his eternal generation, then, is to say that he is the one who eternally is the Son from and only from the Father. Because God is simple and the persons are identical to one another, we distinguish this one from that one by relation origin: who they proceed from. As Stephen R. Holmes puts it after his historical analysis of the tradition in his work, The Quest for the Trinity, “the relationships of origin express/establish relational distinctions between the three existent hypostases; no other distinctions are permissible.”
 John Owen, A brief Declaration and Vindication of the Doctrine of the Holy Trinity, 407, in John Webster, God Without Measure, I:87.
 Basil of Caesarea, Against Eunomius, I.5. While Aquinas does say that the name “Spirit” is not typically a relational expression, it nevertheless identifies a distinct person through relation because the Spirit is of the Father (Matt 10:20) and of the Son (Gal 4:6) (Aquinas, ST Ia.36.1 ad., 2).
 Francis Turretin, Institutes of Elenctic Theology 3 vols. trans. George Musgrave Giger, ed. James T. Dennison, Jr. (Phillipsburg, NJ: P&R, 1992), 1:293.
 Webster, “Eternal Generation,” in God Without Measure, 30.
 Stephen R. Holmes, The Quest for the Trinity: The Doctrine of God in Scripture, History and Modernity (Downers Grove, IL: InterVarsity, 2012), 146.