Warren@23wjohnston
This is why people don’t consider Mormons Christians.
Thomas Aquinas (c. 1225–1274) provides one of the most philosophically rigorous and systematic treatments of the Hypostatic Union in the Summa Theologica (especially Tertia Pars, Questions 1–26 on the Incarnation). Aquinas affirms —two complete natures (divine and human) in one person (hypostasis)—and answers key questions about how this union occurs.
The hypostatic union is a “mixed relation.” The real relation is on the side of the created human nature (which comes to be united to the Word). The divine Person receives no new esse personale (personal existence) or change—only a new relation of His pre-existing personal being to the humanity. The Son now subsists also in the human nature, without any alteration to His divinity.
Because there is one Person, divine attributes (e.g., eternity, omnipotence) and human attributes (e.g., birth, suffering, death) can both be truly predicated of the same Christ (e.g., “God died on the cross” or “the eternal Son was born of Mary”).
The hypostatic union itself is the supreme grace (gratia unionis), uncreated in its divine term but involving a created reality in the humanity. It is distinct from Christ’s habitual sanctifying grace.
Implications in Aquinas’s Christology
• Christ has two wills and two operations (divine and human), united in the one Person.
• The human nature is perfect and complete but dependent for its personhood on the divine Person.
• This union is the greatest possible (after the Trinity itself), fitting for the salvation of humanity: only the God-man can repair the infinite offense against God while truly representing us.
Aquinas’s account is deeply scriptural (e.g., John 1:14, Philippians 2:5–11) and patristic, while using precise metaphysical tools to avoid heresies like Nestorianism (two persons), Monophysitism (one mixed nature), or Adoptionism. It remains foundational in Catholic theology for understanding the Incarnation as a profound mystery that is nevertheless rationally defensible. For primary reading, start with Summa Theologica III, qq. 1–6.
