You might believe in a heresy. Actually, you might believe in two heresies. Do not be alarmed! It’s an accident, I’m sure. Most of us assume these heresies when we describe the union of Christ’s two natures, but it’s up to us to scrub our imaginations and speech of any vestige of them.

The first heresy you might believe is called Apollinarianism. The Chalcedonian Statement on Christology (451 A.D.) confesses that in the incarnation, Christ became truly man, with “a rational soul and human body.” What does that mean exactly? It means that the incarnation was not the moment when the divine Son took on a human body, which was animated by a divine soul. This kind of “incarnation” is what Apollinarians believed and it is unorthodox. And yet, most Christians, if they had to describe the incarnation and the relationship between Christ’s two natures, would sound remarkably Apollinarian. Trevin Wax describes his own experience as an accidental heretic in this way:

I used to believe a heresy. Not intentionally, of course. I discovered my error during my first year of theology classes in Romania. Our systematic theology professor listed heresies of the Trinitarian variety and arrived at Apollinarianism, which he described as “the teaching that Jesus had a divine soul in a human body.” I felt like someone had punched me in the gut. Though this heresy had never been taught in my church, it had somehow wormed its way into my mind as the most logical way to hold Christ’s divinity and humanity together. I’d just assumed that Jesus as “God in the flesh” meant a divine mind/spirit wrapped up in a human body.
Logical or not, it was wrong. I was wrong. This doesn’t mean I wasn’t a true believer in Christ. I had been seeking for years to faithfully follow Jesus, which is why I wound up studying theology in Romania in the first place. My understanding of Christ’s nature was in error, but I was a genuine believer. Once my error was contradicted by the testimony of Scripture and witness of the church through the ages, I corrected my understanding and never looked back.[1]

So, if you—like Wax and many more—have been an accidental Apollinarian, don’t worry: this doesn’t mean you aren’t regenerate or a true disciple of Christ. It does mean, however, that I have just made things difficult for you. By letting you know you’ve been an unwitting heretic, I’ve ruined your happy ignorance and have placed a wonderful burden on you. No longer do you have the luxury of not investigating this question: why is Apollinarianism a heresy? The stakes are now too high for you to ignore that question. You have to answer it now. What I’m about to say is therefore incredibly important, so buckle up.

The short answer to our question is that the Apollinarian incarnation is an incarnation of less than true humanity. Humans, by nature, are composites of body and soul. If Christ only assumed a body, but not a human soul, he didn’t assume a whole human nature. He isn’t truly human. So, the short answer to the question, “why is Apollinarianism a heresy?” is: Apollinarianism doesn’t truly affirm the incarnation of Christ. A longer answer to the question involves our consideration of another related heresy you might believe: monothelitism.

The council that determined monothelitism as a heresy was Constantinople III (681), the third ecumenical council in the city of Constantinople, and the sixth ecumenical council of the early Church. Monothelites insisted that the person of Christ had a single will (monothelatismōs, one will), while the dyothelitism of the dyothelites insisted that Christ had two wills (duothelatismōs, two wills)—one will for his human nature, and another for his divine. These two heresies–Apollinarianism and monothelitism–are interrelated, for the will and the soul are intimately connected. The will is a faculty of the soul, so, to have a soul is to have a will (and vice versa).

This being the case, a human will is essential to a human nature, and so the stakes on the question of whether Christ assumed a unique human will in the incarnation are therefore just as high as on the question of whether he assumed a human soul. Unless we affirm that Christ assumed a human soul with a human will, we are left with two disastrous conclusions—one that jeopardizes the Trinity, and the other that jeopardizes the incarnation (and therefore, our salvation).

On the one hand, if Christ did not assume a human soul with a human will in the incarnation, then we would have to conclude that the Trinity is a composition of three subjects with three wills (and, probably, three centers of consciousness). How so? Well, we would have to conclude from events like Christ’s prayer in the garden of Gethsemane, when he says to the Father, “Not as I will, but as you will” (Matt 26:39), that such an event is the divine will of the Son submitting to the divine will of the Father. If he has only one will, and that one will is his divine will, then the distinction between his will and the Father’s (“not as I will, but as you will”) cannot be the distinction between a human will and the divine will, but rather between two divine wills. But if our Trinity is a composition of three subjects with three wills, we are just a stone’s throw away from tritheism, which is a particular kind of polytheism.

On the other hand, if Christ did not assume a human soul with a human will, then he did not assume a human nature. To put the matter frankly, humans don’t have divine souls or divine wills. So, if you happen to come across a thing that looks like a young Palestinian man—say, praying in a garden and sweating blood, for example—who has a human body but a divine soul and will, you have found something truly remarkable! But it isn’t human.

If this kind of incarnation were the case—the assumption of a human body with a divine soul or spirit—the gospel would be utterly compromised. Why? Because, the perfect obedience of a divine will cannot be attributed to human beings who have human wills! Therefore, the merit of righteousness acquired by the obedience of such a will cannot be imputed the human beings. Our problem is a human problem, and as such, it needs a human solution.

Think back on that moment in the garden of Gethsemane. On who’s account did Christ resolve to obey the divine will and go to the cross? If the monothelites are right, we sort of have to say that he did this on his own account. It is the will of a divine person bowing to the will of another divine person for the sake of divine harmony in the society of divine wills. But if the dyothelites are right (and they are), Christ did this on our account. He submitted a human will to the divine will on behalf of humans. Why? Because it was a human will that broke the law of God and brought sin into the world. It was a human will that racked up the wages of sin. It was a human will that corrupted all subsequent human wills (but one). And therefore, only a human will could pay those wages and restore those corruptions.

[1] Trevin Wax, The Thrill of Orthodoxy: Rediscovering the Adventure of Christian Faith (Downers Grove, IL: IVP, 2022), 50.