On this day in history–October 8, 451–the Council of Chalcedon convened. The council met to resolve theological disputes about how Jesus Christ’s divinity related to his humanity. After several weeks of deliberation, it concluded that Jesus Christ is one person with two natures.
So, following the saintly fathers, we all with one voice teach the confession of one and the same Son, our Lord Jesus Christ: the same perfect in divinity and perfect in humanity, the same truly God and truly man, of a rational soul and a body; consubstantial with the Father as regards his divinity, and the same consubstantial with us as regards his humanity; like us in all respects except for sin; begotten before the ages from the Father as regards his divinity, and in the last days the same for us and for our salvation from Mary, the virgin God-bearer as regards his humanity; one and the same Christ, Son, Lord, only-begotten, acknowledged in two natures which undergo no confusion, no change, no division, no separation; at no point was the difference between the natures taken away through the union, but rather the property of both natures is preserved and comes together into a single person and a single subsistent being; he is not parted or divided into two persons, but is one and the same only-begotten Son, God, Word, Lord Jesus Christ, just as the prophets taught from the beginning about him, and as the Lord Jesus Christ himself instructed us, and as the creed of the fathers handed it down to us.
EWTN, the Catholic media group, has a nice web page with primary resources. It includes the letter of Pope Leo (Rome) to Archbishop Flavian (Constantinople), the Chalcedonian definition (quoted above), and the canons (legal opinions) of the council.