From my understanding, nothing was created at the Incarnation. Jesus always existed, body was created long ago.

Here is a thorough Catholic answer on the nature of body, soul and incarnation on YouTube.

To summarize the video, there is no separate body and soul in a Cartesian sense. This is a fabrication of the modern world that causes a lot of confusion. The human person is a perfect integration of body and soul.

The body is a perfectly good creation; the soul does not live without a body, or at least not "fully". This is why Catholics believe in the resurrection of the body.

I don't imagine you'll find a fulfilling philosophical answer to this, since that would entail explaining Christianity's fundamental mystery. Namely, Jesus Christ is both fully divine and fully human.

http://www.youtube.com/watch?v=qX7q3yDoFD0

Humans are created. Jesus is fully human. Therefore, Jesus is, in the manner relevant to humans, created. If this weren't true, I don't suspect we would say He's fully human.

I couldn't tell you precisely what this means. But, I'd suggest that it hinges on our temporal understanding of creation -- Jesus' human nature is temporal, and therefore created in time as an event in a series of causal events. But, even as sensible as that sounds, I'd be wary of clinging to it as a precise explanation, and I'd be highly wary and skeptical if anyone claimed they could explain this with any precision or certainty. The only folks of which I'm even vaguely aware who have approached a precise understanding of The Incarnation were heretics.

So, the best Catholic answer, I think, to a question about the precise nature of it's fundamental mystery is this:

Christ is fully human and fully divine. He's fully relatable, fully like yourself (or rather, you're fully like Him); but He's also fully foreign (divine). It's a mystery and worth contemplating and praising. But, don't hope for a full understanding or cling to any [likely-false] precise explanation unless you're looking for heresy!

Does the Catholic Church teach that Jesus' human nature was created?

It's perfectly Catholic and Orthodox to say that Christ the man is a creature. Because it's referring to one of His two natures, that's what incarnation about. Unless God became man we wouldn't be able to become deified, theosis. We're created, if He is not becoming us there is no Gospel.

For the Son of God became man so that we might become God.

St. Athanasius, On Incarnation, 54:3, PG 25:192B.

This is the Good News that God whose beginning is not contingent voluntarily condescend to become a human being for us and our salvation. On this is rested the foundation of God's covenant that even though He is in the form of Creator He chooses to be found in the lowest form of a creature. This is the love of God manifested.

A new and unheard of Covenant: God Who is and was, is made a creature.

Pope St. Leo the Great, Sermon XII, On Nativity.

If Christ the man is a creature why Nicene council condemned Arius for calling Him the first creation? Because Arius confused the person of the Logos who is a divine person, begotten not made, with the humanity which He assumed for our salvation. It's the later which we refer to as a creature not the former. A divine person is not a creature, but the humanity which He assumed is a part of creation.

Christ who God and man, is called uncreated and created, impassible and passible.

St. John of Damascus, On the Orthodox Faith, 3:4.

God the Word assumed not only a human body but also a human soul with rational mind. He is inside out identical with us in His humanity. He has a fallen flesh with all of its infirmities, a fallen soul with its psychological defect, and more importantly a fallen rational mind damaged by the fall. So that He can healed our fallen humanity and deified it completely. Unless the Logos assumed a fallen humanity tampered with its weakness and mortality from the Theotokos then there is no salvation. This is possible in Catholic and Orthodox Christology because original sin has nothing to do with guilt. Incarnation is the summit of all mysteries.

What has not been assumed has not been healed

St. Gregory of Nazianzus, Epistle 101:32.

St. Thomas Aquinas wrote extensively on this topic as also confirmed in the Catechism of Catholic Church.

This question is similar with this question.

Thanks to Andrew Leach and his link to Catechism. I'll try to build upon it:

Born of the Virgin Mary, he has truly been made one of us, like to us in all things except sin.

Jesus was (and still is) similar to us in everything except for sin. We humans are God's creation, and we have always been God's creation. Our reason, our feelings, everything that is good about us is created by God (what is bad might be a consequence of the first sin). Jesus took human mind, human heart, human everything (especially what we call "human soul") - everything that is created regarding to us, and so it is likely created regarding to Him too. The Catechism doesn't seem to counteract this view - the "made" (bolded by me) seems to clearly say that human nature (everything that is "like to us") of the Word was created at the moment of His incarnation.

I think this answer is needed to avoid confusions.

The short answer to the title is “yes”: Christ’s human nature is created. (Indeed, it could not be otherwise: the only nature that could possibly be uncreated is the Divine Nature.)

As far as what was created: everything that pertains to that human nature; that is, body, soul, intellect, will, and human operations (actions).

How the Incarnation “works”

Exactly how the Incarnation “works” ontologically is something of a mystery, but Christology gives us some insights.

The key concept is that the Word (that is, the Son, the Second Person of the Trinity) became flesh. In the Words of the Evangelist John:

Καὶ ὁ λόγος σὰρξ ἐγένετο καὶ ἐσκήνωσεν ἐν ἡμῖν.

And the Word became flesh and dwelt [literally: pitched his tent] among us (John 1:14, SBL GNT and ESV).

However, to become something entails a change from possibility to actuality: this type of change can only occur when a creature is involved,* since God is perfectly unchanging and unchangeable.**

For St. John, flesh is a pars pro toto which means, in our terminology, a full human nature: body, human soul, human intellect, and human will. (See Catechism of the Catholic Church 470-477.)

That is precisely how the Nicene-Constantinopolitan Creed describes the Incarnation:

[Πιστεύομεν εἰς ἕνα κύριον Ἰησοῦν Χριστόν, τὸν] κατελθόντα ἐκ τῶν οὐρανῶν καὶ σαρκωθέντα ἐκ πνεύματος ἁγίου καὶ Μαρίας τῆς παρθένου, καὶ ἐνανθρωπήσαντα. (See Denzinger-Hünnermann, 150.)

[We believe in one Lord, Jesus Christ, who] came down from Heaven and became flesh by the Holy Spirit and the Virgin Mary, and became man (my translation of the original Greek).

It is true that the Word, or Son, being God, is completely unchanged by the Incarnation; however, in order to assume a human nature, the Word had to create one.

It follows that Jesus’ human nature, as well as everything that pertains to that nature (the body, the soul, the intellect, the will, the human actions) is indeed created, although the Person of Jesus—Who is Divine—is, of course, not created.

* To be perfectly precise, technically the Incarnation was not a change in something that was already created; it is an act of creation that brings a creature (namely, Christ’s human nature) into being.

** The reason that God is unchanging and unchangeable is that, changes are necessarily provoked by something. Moreover, a change in something must be provoked by something really distinct from itself. (It doesn’t have to be a different substance altogether; one part can change another part, which is what occurs in living things.) In the case of God, however, there is nothing outside of Him that could possibly provoke a change in Him—after all, everything outside of Him owes its very existence to Him. See S.Th. Ia, q. 9, a. 1.

What is the difference between a nature and a soul?

The O.P., in his comments, asks what the difference between nature and soul is.

Nature (which, in this context, is a synonym of essence and substance) answers the question “What?” For example, in the case of Jesus, when we ask “What kind of being is he?” we answer “human and divine.” More precisely, we say that he has a human nature and a Divine Nature.

Without getting too bogged down in detail, most of the time when we use the terms nature and essence, we are referring to the class or—to use more technical terms—the genus or species to which something belongs. For example, I have a human nature because I belong to the species man (and the same thing could be said for Jesus); the dog across the street has a canine nature, because it belongs to the species dog, and so forth. Generally, when we use the term substance, we are referring to individuals, like my neighbor, my brother, and me. These three terms, however, are somewhat flexible, and so sometimes you will see “nature” or “essence” refer to individuals, and “substance” refer to the species, depending on the author.

In any event, to understand what a soul is, it is necessary to understand that material creatures (including man) are always composed of an intelligible component that gives the creature its definition and consistency; and an indeterminate component that is “shaped” (or “informed”) by the former one. Using Aristotle’s terminology, we call the former component the substantial form, and the latter the prime matter.

In living, material creatures (everything from bacteria on up to man), the substantial form can be called a soul (Latin: anima, which we get the term animal), because it gives life (“animates”) the living creature. The prime matter of such a creature (actually, of any material creature, even a nonliving one) we call a “body.”

Of course, in man, the soul does much more than inform and animate the body; it is capable of strictly spiritual actions, such as knowledge and love. However, it is still a soul, inasmuch as it gives life and consistency to a body.

A human soul, therefore, is not a complete nature; it is only part of an (individual) nature, albeit the more noble part.

(If the reader wishes to enter into a really sticky philosophical problem, he can consider the status of a person who has died and is still awaiting the resurrection of the body. Evidently, in such a person, what was his soul is now his entire substance—but that is an anomalous situation, which necessitates the resurrection of the body.)

The fact that Jesus’s complete human nature included a complete human soul, therefore, makes it even clearer that his human nature must be created.

Human nature entails a human soul. When Christ became man, every aspect of His human nature was created:

It is to be remembered that, when the Word took Flesh, there was no change in the Word; all the change was in the Flesh. At the moment of conception, in the womb of the Blessed Mother, through the forcefulness of God's activity, not only was the human soul of Christ created but the Word assumed the man that was conceived. (The Incarnation, The Catholic Faith)

The soul, as I understand it, is the rational substance that moves the body. See:

• http://www.newadvent.org/cathen/09580c.htm
• http://www.newadvent.org/summa/1075.htm