Two early heresies that intrigue me a bit are monoergism and monothelitism. Both basically suggest that while Jesus was both human and divine, he only had one "energy" or one "will." Both were eventually called heresy by the church, but why? Does this this guard against the frankly worse heresy of Nestorianism?. What is so bad about these heresies? What practical effect does it have on one's perception of Jesus if we fall into this trap?
I was trying to explain these two Christological concepts to an unsaved friend of mine, and he said, "Oh, so you're talking about the opposite of demon-possession - that Jesus the man was simply possessed by God."
Stripping that analogy down to its essentials, I think he may be on to something. If Jesus only had one "energy" or one will, then Jesus is essentially a docetist God, only appearing to be man. If his humanity is scooped out and overridden by God's indwelling, then he can't really identify with humanity. To hold to a monothelitist idea is to say that God only "appeared" to be human - but a lobotomized human isn't really all that human.
As such, to hold to this heresy would be to make a mockery of the incarnation. God may have taken on flesh and bone, but not mind and spirit.
And, if he didn't, then he didn't know jack about every Tom. Dick, and Harry out there...
The issue leading up to monothelitism in the late 8th century was caused by multiple diverging Christological viewpoints disagreeing with one another. Severus of Antioch was misread badly by Chalcedonians at that time as explained by Abouna Peter Farrington. There are great differences between Monophysites and Miaphysites, the former collapsed the two natures indistinguishable from one another after union while the later maintain distinction of the two "in thought alone" as St. Cyril interpreted correctly this use in his letter to John of Antioch and his letter to Acacius of Melitene (PG 77,184-201) to Eulogius (PG 77,224-228) and to Succensus (PG 77, 228-245). Miaphysitism is a belief in one compound nature of Logos incarnate. Just as the unity between soul and body in psychosomatic union, the unity between the Logos and His flesh is analogous. From this one nature Christology, Severus developed miathelitism that the Logos has a theandric divine-humane will. To propose that Christ has two wills is to assert that His two natures were never truly united. It's basically a misunderstanding, Chalcedonians see Severian Theandric will jeopardize the distinction of two natures which will lead to either Apollinarianism or Eutychianism while Miathelites see Dyothelitism as dividing the nature of Logos incarnate into two which will lead to Nestorianism. Even though Severian Miathelitism is not heretical, in general sense monothelitism and monergism are indeed heretical. The relationship between Severianism and Monothelitism is like Sabellianism and Modalism, one is a subset of larger theological group.
Monothelitism is a heresy condemned at Third Constantinople (681) because it jeopardize our salvation. The main concern of Christological controversy is not to defend a proper Christology but primarily to defend proper soteriology. St. Maximos the Confessor is the champion of two wills Christology. His main concern for two wills is focused not on how many will Christ possessed but how His wills relate to one another. He uses this relationship between two wills to develop a soteriological relationship between God's will and our human's will in salvation. This is where synergism is established as Orthodox and monergism is condemned as heterodox. At first glance it might not be obvious why from two wills and two activities of Christ Catholic and Orthodox come to synergistic soteriology. But this is the main heart of monothelitism controversy during Maximos' time. How the two wills and energies relate determine the Orthodox soteriology. I don't want to accuse Protestantism for the heresy of monergism, but given a time I'll post a proper question to bring how this relate to soteriology as Maximos intended. Briefly, unless Christ has a truly human will and energy then His incarnation didn't bring healing to our human will and energy. Because Christ has a truly human will and activity, His incarnation healed our will and activity. That way we can receive His grace willfully and actively. I'll also post a question regarding why Pelagius was a monergist and not a synergist as potently seen by Protestant. Maximian Synergy is foundational not only for contemplative Christology but primarily on Synergistic Soteriology. This is the irreconcilable Hallmark that separate synergism in Catholic and Orthodox from monergism in Protestant soteriology with unbridgeable gulf.