Question: "What is monophysitism?"
Answer: Monophysitism is an erroneous or heretical view concerning the nature of Jesus Christ. Its name comes from a combination of the Greek words monos, meaning “one” or ”alone,” and physis, meaning “nature.” According to this view, Christ had only a single divine nature instead of two natures, one divine and one human, as set forth in the orthodox or Chalcedonian position of the hypostatic union of Christ.
This view is also sometimes known as Eutychianism, after Eutyches, a 5th-century leader of a monastery in Constantinople. In response to Nestorianism (the belief that there were two separate persons in Christ, a human person and a divine person), Eutyches taught that Jesus’ humanity was essentially dissolved or obliterated by His divine nature, describing it as being “dissolved like a drop of honey in the sea.” An analogy that might help explain what he meant is a drop of ink put into a glass of water. The result is a mixture that is not pure water or pure ink. Instead, you would have some type of third substance that is a mixture of the two in which both the ink and water are changed in some way. In essence, that is what Eutyches taught about the natures of Christ. He believed that the human nature of Christ was essentially absorbed into His divine nature in a way that both natures were changed to some degree which resulted in a third nature being formed.
Monophysitism or Eutychianism was condemned as a heresy at the Sixth Ecumenical Council in A.D. 680-681. It should be viewed as a heretical teaching because it denies the hypostatic union of Christ and the fact that Jesus was both fully God and fully man as taught in the Scriptures. This is a critical doctrine when it comes to the atonement of Christ. Had Jesus not been fully man, then He could not have been our substitute, and had He not been fully God, then His death could not have made atonement for the sins of all who would believe in Him.