Docetism is the heresy that teaches that Jesus is not human at all. He is completely divine. The Docetists said Jesus just ‘seemed’ to be human. Apollinarianism is another heresy that denies Jesus Christ’s full humanity. In this belief the Logos (the divine part of Jesus Christ) assumed a human body in Jesus, and thus replaced his human soul and mind. But if the divine Word of God took the place of the human mind and soul, Christ the Lord was not completely human. Monophysitism suggests that the humanity of Jesus was overwhelmed or taken up into his divinity.
Some conservative Protestants fall into this trap unawares. They don’t take the labels of “Docetist” or “Apollinarian” or “Monophysite” but their concept of Jesus is that he sort of floated around a few inches off the ground, never soiled his clothes, always had his hair combed, did miracles and because he was God he was always squeaky clean. The “stained glass Jesus” is a popular image of this divine being who only appears to be human. A more popular “man in the street” version understands Jesus’ divinity in super hero terms. Jesus was a wonderworker like Superman, so he must have come here from another planet. Jesus as an alien or an angel. He is really divine but seems to be human like us when he puts on ordinary clothes like Clark Kent.
Why does this matter? Because if those who fall into the trap of thinking Jesus is merely human end up focussing only on making this world a better place, then the docetists float through life so heavenly minded that they’re no earthly good. The docetist Christians are the ones who are unlikely to start a soup kitchen, build a school, get involved in politics or campaign for life. As St Ignatius of Antioch said, “They do not believe the Eucharist is the Body and Blood of Christ and they will not feed the hungry or house the orphans.”
The fathers of the Church realized that one of the safeguards for an orthodox understanding of the Incarnation of Christ was to have the proper understanding of who Mary really is. The Council of Ephesus in 431 approved the term “Theotokos” or “God Bearer” or “Mother of God” for the Blessed Virgin as a support for the orthodox understanding of the incarnation. In Mary the Mother of God we see the two natures combined perfectly: Jesus Christ–true God and true Man. This is why an icon of the Madonna and Child is not just a pretty picture, but always a meditation on the depth and wonder of the incarnation of our Lord Jesus Christ.
The beauty of the Catholic faith is that the true belief about Jesus Christ is not just a theological dogma to be believed in our head. It is a belief we affirm each week in the liturgy as we recite the creed. It is a belief we live through our devotion to the Blessed Mother as we pray the rosary. The incarnation is a belief that comes alive as we celebrate the Eucharist where Jesus Christ, Body, Blood, Soul and Divinity lives for us under the appearance of bread and wine. It is a belief we live as that same Lord lives in us and we in him–remembering that God became man so that we could become like God.
Go here for a quick summary description of the Christological heresies.