The answer
Division in the early churches occurred in 451 A.D. The Oriental Orthodox Churches comprising now the Coptic, Armenian, Syrian, Ethiopian, Eritrean, and the Malankara Indian Churches refused the Council of Chalcedon of 451 AD. The Chalcedonian Council was a very "political" council held in the City of Chalcedon in Asia Minor and dealt with the Heresy of Eutyches, an archimandrite of Constantinople. The Coptic Church was accused of following the teaching of Eutyches, who believed in Monophysitism. This doctrine maintains that our Lord Jesus Christ has only one single nature; the divine nature, and not the human as well as the divine.The Oriental churches have refused and rejected the teaching of Eutyches. They believe that our Lord Jesus Christ is perfect in His divinity and perfect in His humanity, yet His divinity parted not from His humanity not for a single moment nor a twinkle of an eye. The Oriental Churches did not accept the two separate natures of Jesus Christ that was advocated in Chalcedonian Council. The Oriental Orthodox Churches have accepted the formula of faith made by St. Cyril "One nature for the incarnated God the Word" (Miaphysitism). Our belief is Jesus Christ, Him being perfectly divine and perfectly human and these two natures are united together without confusion, without mingling, and without alteration (miaphysitism).
Answered by His Eminence Metropolitan Youssef (opens in a new tab)