The Incarnation of Christ is spoken of in John 1:1,14 which reads, "In the beginning was the Word, and the Word was with God, and the Word was God. ... And the Word was made Flesh, and dwelt among us."
The divine Second Person of the Blessed Trinity, Jesus, assumed a sinless human nature in the womb of Mary with no change to His divine personhood. He is from all eternity the divine Son of God the Father, who 2,000 years ago became the son of Mary when He took on a spotless soul and a sinless body in her womb.
This divine act that began the redemption of sinful man followed the Annunciation commemorated by Catholics on March 25. In Luke 1:31,35 the angel Gabriel announced to Mary, "Behold thou shalt conceive in thy womb, and shalt bring forth a Son, and thou shalt call His name Jesus. ... The Holy Ghost shall come upon thee, and the power of the most High shall overshadow thee. And therefore also the Holy which shall be born of thee shall be called the Son of God."
The term used by the Catholic Church for this union of divine and human natures in the one person of Christ is called the hypostatic union. In Christ, there's one "who" and two "whats." This union is called substantial as distinguished from a merely accidental union, which describes the indwelling of the Holy Spirit within a soul in the state of sanctifying grace. A type of substantial union spoken of by Church Fathers to analogously describe Christ's hypostatic union is the union of an immaterial soul with a material body at conception.
Christ remained the same divine Person after His incarnation. While retaining His divine will He assumed an untainted human will, which was always in perfect conformity with His divine will. In a similar way, Christ retained His omniscient, divine intellect and assumed a human intellect that beheld all created things in the beatific vision.
Explore this mysterious hallmark of Christianity in season one of Armor of God—The Incarnation.