The hypostatic union is the doctrine that Jesus Christ is one Person with two complete, distinct, and unmixed natures — fully divine and fully human. The Council of Chalcedon (451 AD) defined it against four heresies: Nestorianism (two persons), Eutychianism (natures merged into one), Docetism (only divine, no real humanity), and Arianism (not truly divine). The biblical foundation: "In the beginning was the Word...and the Word was God...and the Word became flesh" (John 1:1,14). The two natures are without confusion, without change, without division, without separation — the Definition of Chalcedon. Christ is not a hybrid; He is the God-Man: the eternal Son who added humanity to His deity without diminishing either.
Hypostasis (n., theological): In Trinitarian theology, a distinct subsistence or personal center within the Godhead — the Father, Son, and Holy Spirit are three hypostases (persons) in one ousia (essence). In Christology, the hypostatic union is the doctrine that the divine and human natures are united in the one hypostasis (person) of the eternal Son. The term entered English theological usage through the Nicene and Chalcedonian controversies of the 4th–5th centuries.
The hypostatic union is perpetually threatened from two directions: over-divinizing Christ (denying real humanity — He couldn't really be tempted, really suffer, really not know things) and over-humanizing Christ (reducing Him to an inspired man — a moral teacher, a great prophet, the supreme example). Both errors destroy the gospel: a Christ who is not fully human cannot represent humanity; a Christ who is not fully divine cannot save it. Modern "progressive Christianity" consistently erodes the divine nature, reducing Christ to a social reformer. Some Word of Faith theology swings the other direction, over-spiritualizing Christ to the point that His humanity becomes ceremonial rather than real.
Greek:
ὑπόστασις (hypostasis, G5287) — substance, underlying reality, person
← ὑπό (hypo) — under
+ στάσις (stasis) — standing, position
← ἵστημι (histēmi) — to stand, to place
Used in Hebrews 1:3: "He is the radiance of the glory of God
and the exact imprint of his nature (hypostasis)"
Also in Hebrews 11:1: "faith is the substance (hypostasis) of things hoped for"
Chalcedon (451 AD) technical vocabulary:
φύσις (physis) — nature (two natures in Christ)
ὑπόστασις (hypostasis) — person (one person in Christ)
οὐσία (ousia) — essence/being (one divine essence in Trinity)
πρόσωπον (prosōpon) — face/person (synonym for hypostasis)
hypostasis (ὑπόστασις, G5287) — substance, nature, person; used in Hebrews 1:3 to describe Christ as "the exact imprint of his (God's) nature" and in 11:1 for "substance" of faith.
morphē (μορφή, G3444) — form, outward expression of inner nature; used in Philippians 2:6–7 — Christ was in the "form of God" and took the "form of a servant."
• John 1:1,14 — "In the beginning was the Word...the Word was God...and the Word became flesh and dwelt among us."
• Colossians 2:9 — "In him the whole fullness of deity dwells bodily."
• Philippians 2:6–7 — "Though he was in the form of God...he emptied himself, taking the form of a servant, being born in the likeness of men."
• Hebrews 4:15 — "We have a high priest who...has been tempted as we are, yet without sin."
• 1 Timothy 2:5 — "There is one mediator between God and men, the man Christ Jesus."
• "The hypostatic union is not a puzzle to be solved but a mystery to be worshipped — God became what we are so we could become what He is."
• "Jesus was tired enough to sleep in the boat (Mark 4:38) and powerful enough to stop the storm — that's the hypostatic union in one verse."
• "A Christ who is less than fully God cannot save you. A Christ who is less than fully man cannot represent you."