Who is Jesus Christ? He is the Second Person of the Blessed Trinity, conceived by the Holy Spirit and born of the Virgin Mary. He is true God and true man....
It is important to understand that Jesus had a human soul. He was also endowed with true human knowledge, which always worked in harmony with the divine wisdom to which Jesus' knowledge was united. Jesus also possessed a true human will, which always cooperated with his divine will....
The Council of Ephesus (AD 431)...professed that Mary is the Mother of God, the Theotokos (Birth-giver of God; sometimes translated as "God-bearer"). Jesus Christ is the divine Son of God who became man in the womb of Mary. The one who was born of Mary is the same one--the same person--who has existed with the Father and the Holy Spirit from all eternity.
Understanding that Jesus is both fully human and fully divine is very important. The Church has consistently defended this teaching against attempts to present one or the other as somehow less. If the Crucifixion and Resurrection were events that involved God only, then we are not saved. If Jesus was not divine, he would have been just another good man whose death and Resurrection would not have saved us. It is necessary to believe that the mystery of the Incarnation means that Jesus was both fully God and fully man.
--from United States Catholic Catechism for Adults, pp. 81-83