Final answer:
At the Council of Chalcedon, the nature of Christ was a subject of disagreement, leading to a schism within the Christian Church in Egypt.
Step-by-step explanation:
At the Council of Chalcedon in 451, disagreements arose about the nature of Christ, specifically whether he was fully divine and human (the Dyophysite position) or if his humanity was inseparable from his divinity (the Monophysite position).
Some people rejected Pope Leo's letter at the Council of Ephesus because they disagreed with the decision to condemn the Monophysite position as heretical. The later Council of Chalcedon attempted to resolve these disagreements by affirming the Dyophysite position, which led to a schism with the Christian Church in Egypt, known today as the Coptic Church.