Chalcedonian Creed 451AD
The Fourth Ecumenical Council was called by the emperor Marcian in 451AD. It originally convened in Nicaea but later transferred to Chalcedon, so as to be closer to Constantinople. The resulting Creed, known as the Calcedonian Creed, or the Definition of Chalcedon, addressed the heresy of Monophysitism (Gk - one nature) regarding the nature of Christ.
The Council of Nicaea 325AD previously defined Jesus being fully divine and fully human. It did did not; however, state how Christ could be both divine and human, or how the divine and human were related within His person. This led to the Christological controversies of the fourth and fifth centuries which the Council of Chalcedon attempted to address. To summarize, the produced doctrine stated that two natures, one human and one divine, are united in the one person of Christ. The Council further stated that each of these natures, the human and the divine, was distinct and complete.
The western churches readily accepted the creed, but some eastern churches still held to the Alexandrian formula of the oneness of Christ’s nature as the incarnation of God the Word (that is, His Divinity and Humanity are united in one nature). In 681AD, the Third Council of Constantinople attempted to restore the harmony between the churches caused by this controversy.
Modern Translation
We, then, following the holy Fathers, all with one consent, teach men to confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood;
truly God and truly man, of a reasonable {rational] soul and body;
consubstantial [co-essential] with the Father according to the Godhead, and consubstantial with us according to the Manhood;
in all things like unto us, without sin;
begotten before all ages of the Father according to the Godhead, and in these latter days, for us and for our salvation, born of the Virgin Mary, the Mother of God, according to the Manhood;
one and the same Christ, Son, Lord, only begotten, to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, inseparably;
the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person and one Subsistence, not parted or divided into two persons, but one and the same Son, and only begotten, God the Word, the Lord Jesus Christ;
as the prophets from the beginning [have declared] concerning Him, and the Lord Jesus Christ Himself has taught us, and the Creed of the holy Fathers* has handed down to us.
*Note - "Creed of the holy Fathers" refers to the Nicene Creed of 325AD.
Traditional Translation
Following, then, the holy fathers, we unite in teaching all men to confess the one and only Son, our Lord Jesus Christ. This selfsame one is perfect both in deity and in humanness; this selfsame one is also actually God and actually man, with a rational soul {meaning human soul} and a body. He is of the same reality as God as far as his deity is concerned and of the same reality as we ourselves as far as his humanness is concerned; thus like us in all respects, sin only excepted. Before time began he was begotten of the Father, in respect of his deity, and now in these "last days," for us and behalf of our salvation, this selfsame one was born of Mary the virgin, who is God-bearer in respect of his humanness.
We also teach that we apprehend this one and only Christ-Son, Lord, only-begotten -- in two natures; and we do this without confusing the two natures, without transmuting one nature into the other, without dividing them into two separate categories, without con- trasting them according to area or function. The distinctiveness of each nature is not nullified by the union. Instead, the "properties" of each nature are conserved and both natures concur in one "person" and in one reality {hypostasis}. They are not divided or cut into two persons, but are together the one and only and only-begotten Word {Logos} of God, the Lord Jesus Christ. Thus have the prophets of old testified; thus the Lord Jesus Christ himself taught us; thus the Symbol of Fathers* has handed down to us.
*Note - "Creed of the holy Fathers" refers to the Nicene Creed of 325AD.