We confess that Jesus Christ is both true man and true God. The earliest Christians already drew this conclusion, based on the testimony of the Bible. They called Jesus the theanthropos, the “God-Man”. When in the early 300s Arius denied that Jesus is God, the church famously stated in the Nicene Creed: he is “true God of true God, …, of one substance with the Father;” but also: he “was made man.”
The wonderful fact that Jesus has two natures—divine and human—is known in theology as the hypostatic union. But this hypostatic union is difficult to understand. Over the centuries, there were many failed attempts to pinpoint the precise relationship between the divine and the human in Jesus Christ. Ultimately, the ecumenical council of Chalcedon in 451 AD adopted a “Definition” dealing with this question. It does not explain precisely how the hypostatic union works—because we cannot comprehend it—but it shows the Biblical lines within which our understand must stay.
Although the Definition of Chalcedon is not listed among the creeds our churches adopted, much of its substance is found in the Athanasian Creed and in the Belgic Confession (see e.g. BC art. 9, 18, 19). Here follows a translation of the Chalcedonian Formula:
“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 rational soul and body; coessential 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, without confusion, without change, without division, without separation; 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.” (from prca.org)
Especially the highlighted phrase in this Definition warns us not to mix up the natures of Christ unduly (“without confusion”), but at the same time also not to split Christ into two Persons (“without division, without separation”).