Jesus Christ: True God and True Man (Aquinas 101)

The Christian faith professes that Jesus Christ is true God and true man. That is, that the eternal Son of God, the Word of the Father, entered into time and assumed a complete human nature to himself, thus truly becoming man for us and for our salvation. This is the central claim of Christianity.

It’s a truth that’s at once beautiful, high and mysterious. St. Thomas Aquinas meditated deeply on it. In the third part of his Summa Theologiae, he sought to show its deep intelligibility and its coherence. Now, Aquinas always takes his starting point from the divine Revelation about Christ contained

In Sacred Scripture as understood according to the Church’s apostolic Tradition, including the great church councils that excluded false interpretations of what Scripture teaches about Christ. It helps to start by reviewing these early errors about Jesus so that we can better understand the mystery of the Incarnation.

In the early centuries of the Christian age, everyone agreed that God can neither change nor suffer, but this raises a big question. How can we affirm this while also saying that the Son of God became man and suffered on the cross?

One erroneous response to this was offered by the heresy called docetism, which comes from the Greek word meaning to appear or to seem. On this view, Jesus only seemed to be a man and therefore he only appeared to suffer.

This early heresy was ruled out very quickly by the Church as undermining what Scripture teaches, that Jesus really was human, he really did suffer and die on the cross for our sins, and he really rose from the dead in his human body on the third day. Another early error was adoptionism.

It claimed that Jesus was a man adopted by God the Father. Some thought this happened when he was baptized in the Jordan River, and that from that point onward, he comes to be called the Son of God. The Church rejected this too.

The Son of God did not assume a man who was already existing, rather the Son of God pre-existed in eternity with the Father and then became incarnate in the womb of the Virgin Mary in time. Arius and the Arian heresy erred in a different

Direction by saying that the Son of God was not truly God, but was rather a creature. A high creature, but still a creature. The church condemned this at the Council of Nicaea in 325 AD, where it professed that

The Son of God is, ” … true God from true God, begotten not made, consubstantial with the Father.” That is, of the same being or substance as the Father. After this, the error of Nestorianism arose. Although there’s dispute today about what Nestorius actually taught, the Nestorian heresy

Seeks to insulate the eternal Son from change and suffering by distinguishing the man Jesus Christ from the divine Word. Nestorianism claimed that the Word dwells within the man Jesus according to grace, but that we can’t necessarily attribute to the Word all of the things that we say about Jesus.

For example, that he was born of Mary or that he suffered on the cross. The church condemned this at the Council of Ephesus in 431 AD; and two decades later, the Council of Chalcedon formulated a definitive statement of the Church’s belief in Jesus Christ.

Jesus Christ is one person who is true God and true man. To put this more precisely, the eternal Son of God is a divine person who assumes a complete human nature in the womb of the Virgin Mary body and soul, having a human mind and a human will.

From that point forward then, he subsists in two natures, the divine nature and an individual human nature. These two natures are united in the subsisting person of the Son, and this is called the hypostatic union, which comes from the Greek word hypostatis, which means a subsisting individual. Aquinas studied these early church councils

Carefully, and he provides an insightful synthesis of their teaching. The key points that he makes can be summarized like this. First, when we say that this man Jesus Christ really is God, we’re making a metaphysical claim about being.

Its foundation is the hypostatic union; that is, that the two natures of Christ are truly united in his person. In fact, Aquinas says that person is a metaphysical term. It means a subsisting individual of a rational nature.

So the union of man and God in Christ is not simply by indwelling or by habitual grace, rather the divine person of the Word now subsists in, or according to, a complete human nature in addition to the divine nature. Second, the union of the divinity and humanity

In Christ doesn’t change either of these natures. The Son is and remains fully and truly divine, eternal and unchanging as God. Likewise, the humanity of Christ is not absorbed into the divinity, it remains human, which means it’s in time, it can change, and it can and does suffer.

Next, this union is completely unique. It’s not an accidental union like the way a quality exists in a substance, and it’s not like a human person who puts on new clothes. Aquinas actually uses an analogy to what we would today call an organ transplant. Suppose you received a transplant of a third arm.

Now this arm left to itself won’t continue to exist for very long on its own disconnected from a body; but once it’s joined to your body, it begins to exist by your existence or the subsistence of your soul.

That is, your soul is the principle of your body’s life, and now it is animating this new addition to your body, this transplanted arm. Aquinas doesn’t think this is a perfect analogy, especially because an individual human nature is the sort of thing that normally does exist

On its own, and that’s what the Son is assuming. But this analogy does help us understand that in Christ the eternal Son doesn’t change in his divinity; rather, he joins an individual human nature to himself so that from its first

Moment, that human reality subsists according to the very being of the person of the Son. The man Jesus is the Son in person. St. Thomas follows the Church Fathers in saying that because of the hypostatic union, the humanity of Christ is like an instrument joined to the person of the Word.

Think of a saw in the hand of a carpenter. The saw provides something important. Because it’s iron, it can be sharpened and can cut wood. That’s something that the carpenter’s hand by itself can’t do. But the saw isn’t able to cut wood without the activity of the carpenter.

So by being taken up into the carpenter’s activity, the saw now shares in what the carpenter does. The carpenter is making a bench. This is also what happens in the Incarnation. The eternal Word of God takes up a human nature; and now it’s not a separated instrument like

A saw is separated from your hand, but rather an instrument that is conjoined in the person of the Son himself. In his divine nature as a pure spirit, the Word of God cannot be seen by human eyes. He can’t walk on the shores of Galilee. He can’t lay hands on a blind man.

Above all, in his divine nature, he cannot suffer and die. But by the hypostatic union, the Son assumes a human nature as his instrument, and he uses that humanity so that he himself acting in person can do all those things as man. This is a wonderful truth of the Christian faith.

The Word of God is not only present in the world as God, but joins a creature to himself in the most intimate of all unions. In doing this, he raises up our fallen humanity and creates it anew. Indeed, he lifts it unsurpassingly high to the very divinity itself.

As St. Thomas says following Pope St. Leo the Great, “This is a marvelous exchange. He humbled himself to share in our humanity so that we might be raised up to share in his divinity.” For readings, podcasts and more videos like this, go to

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