The theological dogma that the Person of Jesus Christ was fully God and fully Man. This holds that Christ's humanity and divinity are not mixed, but are united without loss of separate identity. The doctrine of the hypostatic union is an attempt to explain how Jesus could be both God and man at the same time. It is ultimately, though, a doctrine that human minds are incapable of understanding fully. It holds that there is no mixture or dilution of Christ's Human or Divine nature, and that He is one united Person. The doctrine of the hypostatic union was made dogma at the Council of Chalcedon in 451 A.D., in response to the need to clearly define the Church's position in the face of heretical teachings, and further articulated at the fifth general council at Constantinople in 533 A.D. The latter council declared that the union of two natures is real (against Arius), not a mere indwelling of God in a man (against Nestorius), with a rational soul (against Apollinaris), and that in Christ’s divine nature remains unchanged (against Eutyches).
Doctrines which stand in opposition to the doctrine of the hypostatic union include: Docetism, Arianism, Apollinarianism, Ebionism, Eutychianism, and Nestorianism. The Church eventually declared all of these doctrines heresies by the end of the Sixth Century A.D.