I've heard Jesus Christ described as "fully man and fully God." What exactly does that mean?
The statement that Jesus is “fully man and fully God” arose in the early church. It was based on the comprehensive reading of Scripture regarding Jesus’ natures and person. We know that Mary’s firstborn son “grew and became strong, filled with wisdom, and the grace of God was on him” (Luke 2:40). Like other humans, He hungered (Matt. 4:2), thirsted (John 19:28), tired (John 4:6), suffered (1 Peter 3:18), and died (Mark 15:37).
Jesus also clearly showed Himself to be God. Paul writes, “For in Christ all the fullness of the Deity lives in bodily form” (Col. 2:8). According to the consistent testimony of His disciples, Jesus exercises God’s wisdom (1 Cor. 1:30), power, and authority (Heb. 1:3), despite sharing in the frailties of the human existence.
The Council of Chalcedon (451 A.D.) defined Christ as having “two natures: inconfusedly, unchangeably, indivisibly, inseparably.” He is neither “half man and half God,” like a mythological demigod, nor two symbiotically related persons: one human and one divine. He who eternally was “in very nature God” (Phil. 2:6) “became flesh and made his dwelling among us” (John 1:14), bearing the “exact representation of his being” (Heb. 1:3) as the “image,” or "precise physical representation,“ of the invisible God” (Col. 1:15). Jesus is divine without defect and human without exception.
The fact that Jesus was both fully man and fully God is directly relevant to our salvation. As a man, Jesus is able to identify with our experiences. “Because he himself has suffered when he was tempted, he is able to help those who are being tempted” (Heb. 2:18). Yet, because He is fully God, we know that He has overcome the world (John 16:33). On the cross and through the tomb, Jesus represents humanity to God and God to humanity by taking the place of sinful humans who owe an insurmountable debt to our holy God. He then rose as our Savior, conquering sin and death.
We can declare with great confidence that “there is one mediator between God and mankind, the man Christ Jesus, who gave himself as a ransom for all people” (1 Tim. 2:5–6). The fellowship between humanity and God, which was lost in the Garden of Eden, has been restored in Jesus Christ who is fully man and fully God.