With the birth of Christ on the minds many Christians this time of year, this is an ideal time to make sure that we are thinking and speaking rightly about the child born to Mary. What we believe about Christ’s incarnation is a test-case for whether or not we are truly Christians. It’s that important.
Before you panic, this doesn’t mean our eternity depends on our ability to articulate precisely what it means for Christ to be both God and man. However, our view of the incarnation is inseparable from our view of the gospel. In fact, according to the apostle John, to deny Christ’s incarnation is to align with the spirit of the antichrist:
By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, and every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you heard was coming and now is in the world already.” (1 John 4:2–3)
Given the stakes, and so that we may rightly rejoice in the wonder and mystery and glory of Christ’s incarnation, we need to make sure we understand the Bible’s teaching on this all-important question: Who is this child?
Two Bedrock Truths to Affirm
Admittedly, the incarnation is not easy to wrap our minds around, but there are some biblical guard rails that can keep us on the gospel path and prevent us from straying into heresy. Thankfully, we are not the first Christians to think through this issue.
For centuries, the church (guided and illumined by the same Holy Spirit we possess) has thought, written, and defended what Scripture means (and doesn’t mean) when it says that “the Word became flesh” (John 1:14). Along with our spiritual forefathers, then, we must hold together the following two biblical truths.
1. Christ was fully God.
Acknowledging Christ’s deity is fundamental to what it means to be a Christian. After all, Christians are those who confess Jesus as “Lord” (Rom 10:9). The same John who told us that “the Word became flesh” (John 1:14) told us, in the same breath, that “the Word was God” (1). There’s not enough space to list all the other verses and passages that witness to Christ’s deity. A few will suffice:
For in him [Jesus] all the fullness of God was pleased to dwell. (Colossians 1:19)
He [Jesus] is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power. (Hebrews 1:3)
. . . waiting for our blessed hope, the appearing of the glory of our great God and Savior Jesus Christ . . .” (Titus 2:13)
It’s appropriate that Christ, who was fully God, was worshipped as a child (Matt 2:11) and then later during His earthly ministry (Matthew 14:33). He continues to be worshiped today by His followers across the globe.
2. Christ was fully human.
Although the Christ child was fully God, He was also human. And His humanity was not like a robe that could be discarded. He was fully human, like us, and though He never sinned, He experienced temptation and weakness with respect to His humanity (Hebrews 4:15). Jesus became hungry, tired, thirsty, and sorrowful. Mary had to feed him, change him, and teach him the Hebrew alphabet. And this is good news, because, as humans, we needed a fully human mediator:
“For there is one God, and there is one mediator between God and men, the man Christ Jesus . . .” (1 Timothy 2:5)
Because he shares our humanity, Christ is able to reconcile us to God.
Two Landmines to Avoid
For those who affirm that Christ is both fully God and fully man, there are still some landmines to avoid. Here are two errors that have surfaced in different forms in the centuries since Christ’s birth.
1. Christ is not a “mixture” of divinity and humanity.
One strand of false teaching the church had to contend against in the early centuries was the idea that Christ had only one nature. In this view, the child born to Mary was something like a “mixture” of divinity and humanity. 
Perhaps some Christians today think of Jesus in this way, even if they haven’t worked out the details. Jesus is not quite human and not quite divine but rather some kind of hybrid. But this is not what the Bible teaches, and it is not what Christians have historically affirmed. Christ is one person with two distinct natures.
2. Christ’s divinity and humanity cannot be divided.
Although Christ’s divine and human natures are distinct, they cannot be divided. They were forever joined at Christ’s incarnation.  He was not born as a human and then later elevated to a divine status. The child born to Mary was the Son of God, just as the angel promised:
“The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy—the Son of God.” (Luke 1:35)
No wonder Elizabeth called Mary “the mother of my Lord” (Luke 1:43).
Thankfully, we won’t have to explain all the nuances of Christ’s divine and human natures in order to get into heaven. Where we still have questions, we can trust in God’s infinite wisdom and power and goodness. The child born to Mary was (and still is) fully God and fully man, and herein lies our sure hope of salvation.
 This error was known as Monophysitism, or Eutychianism.
 One strand of this error––dividing the two natures of Christ––is known as Nestorianism. For more on some of the historical development of the doctrine of Christ’s person, see Michael Horton, Pilgrim Theology, pp. 177–183.