The hypostatic union refers to the mystery that in the one Person of Jesus Christ, two distinct natures—the divine and the human—are inseparably united without confusion, mixture, or division. Through this union, Christ is truly God and truly man, remaining one Mediator for our salvation.
The 1689 LBCF Ch.8 §2 confesses that the Son of God took upon Himself human nature so that two whole, perfect, and distinct natures—the Godhead and the manhood—were inseparably joined together in one person, without conversion, composition, or confusion. This aligns with the historic Chalcedonian Definition, which declares that Christ is one and the same Son, acknowledged in two natures unconfusedly, unchangeably, indivisibly, and inseparably. As Scripture testifies, "For in him dwelleth all the fulness of the Godhead bodily" (Colossians 2:9).
Because Christ is one Person, the properties of both natures are attributed to Him as the Mediator. The 1689 LBCF Ch.8 §7 and the Westminster Confession of Faith Ch.8 §7 explain that while each nature does what is proper to itself, Scripture sometimes attributes the property of one nature to the person denominated by the other. John Calvin, in the Institutes of the Christian Religion Ch.14 §1, describes this as the communicatio idiomatum (communication of properties), where, for example, it is said that God purchased the church with His own blood (Acts 20:28). This is not because the divine nature has blood, but because the Person who shed it is both God and man.
This doctrine is not merely academic; it is the ground of our salvation. As stated in the Belgic Confession Art. 19, our salvation depends on the reality of His humanity, for He became man that He might die for us according to the infirmity of His flesh, yet remained true God to conquer death. Christ is the bridge between God and man, and because He took our nature, we who were "more sinful and flawed than we ever dared believe" are "more accepted and loved in Christ than we ever dared hope." As it is written: "For there is one God, one mediator also between God and men, himself man, Christ Jesus" (1 Timothy 2:5).