That God was pleased to take not only our humanity, but also the humanity of Jesus united to the Person of God the Son for all eternity, is the core of the Christian faith. "And the Word became flesh and made his dwelling among us, and we saw his glory, the glory as of the Father's only Son, full of grace and truth" (Jn 1:14). John reveals the primary mission of the Son of God in this beloved passage, that God became flesh - that in Jesus Christ, He took on human flesh like us in every way, except for sin (see Heb 4:15).
Why is this doctrine so important? Simply put, without the Incarnation, mankind would have remained in sin and therefore, separated from God. Because of the Incarnation, salvation comes to us through the cross and resurrection of Jesus, but it also opens the way to a greater inheritance. What exactly is the incarnation?
The Incarnation is the mystery and the dogma of the Word made Flesh. The Latin Fathers, from the fourth century, make common use of the word; so, Saints Jerome, Ambrose, etc. The Latin incarnatio (in: caro, flesh) corresponds to the Greek sarkosis, or ensarkosis, which words depend on John (1:14) kai ho Logos sarx egeneto, "And the Word was made flesh". These two terms were in use by the Greek Fathers from the time of St. Irenaeus. The verb sarkousthai, to be made flesh, occurs in the creed of the Council of Nicaea (cf. Denzinger, "Enchiridion", n. 86). In the language of Holy Writing, flesh means, human nature or man (cf. Luke 3:6; Romans 3:20). Man is called flesh to emphasize the weaker part of his nature. When the Word is said to have been incarnate, to have been made Flesh, the Divine goodness is better expressed whereby God "emptied Himself . . . and was found in outward bearing like a man" (Philippians 2:7); He took upon Himself not only the nature of man, a nature capable of suffering and sickness and death, He became like a man in all save only sin.
The early Church Father St. Athanasius enunciates it in his famous phrase: "For the Son of God became man so that we might become God." This is known in the Church as the doctrine of divinization: that the Incarnation effects not just a restoration of our relationship with God, but that because God took on our humanity, it has been raised to a greater dignity than before. The fact of the Incarnation now lifts our human nature to greater heights than it ever hoped for. As St. Paul says, "You received a spirit of adoption, through which we cry, 'Abba, Father!'" (Rom 8:15).
Because God has become man, man has been lifted up into the very life of God. This all happens in Christ and is made possible through his body, the Church. At baptism, we are grafted to the Body of Christ. Thus, where Christ is, there we are also. We, by baptism, are lifted into the very life of God, into the very heart of the Trinity! We cry out to God our Father, enabled to do so in Christ and by the power of the Holy Spirit. That same Spirit animates Christ's body here on Earth: the Church. It is in the Church that we participate in Christ's relationship with the Father: we cannot have Christ without his Church, for it is his Church that makes us members of his body. It is in the Church that — through the liturgy and sacraments — we participate in the very life of the Trinity.
Why does the Incarnation matter? Because by it, our humanity is lifted into the very life of God, we are lifted higher than the angels, and given a dignity greater than in the first creation. Because of the Incarnation, we are made close to God because we are made his sons and daughters through the Son.