Monophysitism

Cyril of Alexandria liked to wear half a beehive on his head.

Monophysitism, in Christianity, is the assertion that Jesus Christ is a single package.

While a few Christians believe that Christ was schizoid, most pondered the details of His Oneness.
These questions have baffled clerics throughout the ages.
(However they were somewhat preoccupied defending their palaces against advancing Huns and Goths at the time so it was something of a back burner issue really).

 In the 4th century, the issue was whether Jesus and God were the same person but in different guises (leisure suit vs. birthday suit). The ancients did not have Clark Kent vs. Superman as a model to work from., and even if they did, there were no phone booths available for Jesus to put on his cape and leotards. The dilemma led to a lot of bitter arguments between 'Nicene' Christians and the not-so-Nicene Arian Christians. This fight eventually saw the Arians booted out the door.

Then the argument became about Jesus himself.
How does a divine being manifest himself when it came to balancing
his spiritual know-it-allness and physical shell?
Where does one stop and the other start?
The term Monophysite arose in the 5th century, an era of decline, confusion, and general beastliness when the Roman Empire broke apart.
The Christians — who had spent previous centuries hiding in catacombs or graveyards — had plenty of time to ponder the exact nature of their favorite deity suddenly had  their status change when Emperor Constantine  shouted "Olly olly oxen free" which gave them the all-clear to emerge and to get on with spreading smallpox.
Yet one nagging problem remained: They had still not nailed down the exact nature of Jesus Christ.

Some of Cyril's supporters chanting and carrying on.

Theologians, more or less wasted conclave after conclave retrospectively analysing how many pinheads could dance on an angel...but they also wondered what was going on inside Jesus's head and body to no avail. In 428, the arguments shifted to the position of the Virgin Mary.
Bishop Nestorius the Enormous of Constantinople was sure God as Jesus had no need of a woman's body to pop out of. (He also was certain Justinian Beaver was a talented musician...so they did not call him "Nestorius the Infallible"...not even once.)
Nestorius proclaimed  Jesus had no need of a childhood; so Mary must have given birth to  a full grown Manly Jesus. (While the human birth canal is indeed elastic, this was a real stretch.)
This all seemed to suggest two distinct entities for Jesus: Godhead Jesus and Manhood Jesus.
That meant the virgin birth was of twins.
 This theological position was named 'Nestorian Christianity,' in contrast to the formerly unanimous 'Nicene' version. The Nestorians insisted they were still also Nicene even though they disagreed on the nature of Jesus.

Bishop Cyril of Alexandria,  after receiving a secret report from Constantinople from a holy insider called Eutyches. wanted to grab Nestorius by the cassock and throw him against a statue of Mary. Cyril was no weak chinned, clap-happy televangelist. If you disagreed with him on anything, he would send around his monks to beat the crap out of you.
He had shown no mercy when dealing with pagans and women teaching geometry.
Hypatia, the great philosopher/pole dancer had been stripped of her clothes and then of her flesh by the blood-lusting Cyril crowd.
 Cyril got his kicks in against Nestorius and his supporters — twelve kicks, in fact, called the 'anathema.' The dozen technical missteps about Christianity that could land you in boiling hot water went like this—

1.  Virgin Mary is the mother of God. Hers was a divine pregnancy.
2. Anyone suggesting She wasn't gets a boot to the face.
3. Anyone suggesting the boot is too extreme gets the boot to the mouth.
4. Jesus had only one tongue.
5. Jesus had only one change of underwear.
6. Everyone must not change their underwear to honor Jesus.
7. Everyone must buy Cyrus brand underwear...no exceptions.
8. Everyone must wear their Cyrus brand underwear on the outside so the church can check.
9. To further punish Hypatia, all future quartets will be any number of people except 4.
10. Anyone who disagrees will burn in hell
11. Jesus is the ho ho holiest of all ghosts & spooks
12. Whosoever shall not confess: let him be anathema.

When people started to tire of these dopey arguments, Cyril published a further annoying decree.
 The Five Tomes, The Collected Letters and Post-it notes... more or less repeating everything again.
 He was going to bury Nestorius.  Cyril became so involved in the dispute that, in 431,
 he went beard-to-beard with Nestorius at the WWF Council of Ephesus.
The fix was already in, as the latter's supporters
had been given the wrong date and address for the match.
Cyril declared Nestorius's views on Jesus as heretical as he pinned him to the mat
and then he glued googly eyes on all his posters around town.
He even defied Emperor Theodosius II The Acrimonious,
when the latter put him under house arrest and threatened to sew Cyril's lips together.
Nestorius got the sack and later got a room (but no key) at an Egyptian monastery.

Eutyches finds a handy pedestal with his name on it to espouse his view of Jesus.

Cyril stayed 'on-side' with respect to orthodox Nicene Christianity — until going down to the pitch for good in 444. The next patriarch was Dioscorus, whose ally in Constantinople, Eutyches, brought a new improved Christian formula: Jesus didn't have two natures, but only one nature, which had simply absorbed the human stuff. So the team adopted the term 'Monophysite.' They denied the Nestorian position much more gracefully than heretics of earlier centuries, who believed that Jesus was an invisible plant, and what the people of Nazareth saw was merely a hologram.

The Christian world was quickly moving to a three way split
or four if you counted the Arians as well
(yes by this time a few did except the German barbarians).
  They could be described like this:

• Nestorians: One person, two realities, two natures.
• Catholics: One person, one reality, two natures.
• Monophysites: One person, one reality, one nature.
• Certs breath mints: It's two — two — two mints in one.

The actual Greek term for reality was 'hypostasis'.
You can also say 'substance,' but people will think about a drug bust.
It was a tag-team match to the death, as all sides invoked God to
support their positions and smite their rivals.
Once the idea of compromise was ruled out, the most extreme views prevailed.
This was seen at the Second Council of Ephesus of 449 and the Council of Chalcedon of 451 where the Catholics came out on top and rewrote the Christian creed to exclude both the Nestorians and Monophysites.
In fact, Nestorius nimbly denied he was a 'Nestorian' and Patriarch Dioscorus of Alexandria denied he was a 'one reality/one nature' bishop, but everyone had seen, heard, & read his positions and knew his pants were in flames at this point.
Dioscorus was removed as Patriarch of Alexandria and pro-Chalcedonian Christian Proterius replaced him till the later was in turn knocked down and killed by a mob of thuggish monks.
Proterius was replaced by the ardent Monophysite Timothy the Weasel, a tiny man with the propensity to bite your ankles and, if he found a chair, would go straight for the throat.
The Nestorians quit the Roman Empire to move further East.
The Monophysites remained firmly entrenched in Egypt and Syria with pockets of support elsewhere.
In Western Europe and most of North Africa, the population remained Catholic, though for a time under the rule of Arian Christians like the Goth "Theodoric the Greasy" who was naturally hostile to all forms of Nicene Christian baking.
To try to keep the Eastern Roman Empire intact, further church councils searched for a magic formula to satisfy both Catholics/Orthodox and Monophysites. All failed and instead sliced-and-diced it finer, including a new division called 'Monothelitism' (or 'Monophysitism Lite'), a 'compromise' that said Jesus had two natures but only one will. (Of course, this led to corrupted manuscripts often mischievously rendering this as 'one willy.')

St. "Ayy" Macarena (center) shows the other girls trendy arm and hand movements
known today as special pleading.

Over time, as with all schisms, there were plenty of further attempts to portray one side as angelic whilst any other interpretation were gyrating to the devil's song book.
In the 7th century, these divisions became unbridgeable — and in any case, unenforceable, as North Africa fell to the armies of Islam. To the Muslims, all sects of Christianity were equally infidel.
Rival Christians, whether Catholic, Monophysite or Nestorian, were left to use 'special pleading' with a caliph or emir to save their their necks. A word of advice. It's unwise to go into a Coptic Christian Church and call them Monophysites. You will get a whack on the head from a bishop or monk manhandling a crozier.