Heresy is a doctrinal error that affects a fundamental of Christianity. Since Jesus Christ is the foundation of Christianity, getting the teaching of Christ wrong can quickly devolve into heresy.
The good news is that getting Christ right is not hard. The bad news is that there are many ways to get him wrong.
This is evident when people talk about the incarnation of Jesus Christ. Incarnation refers to Christ putting on flesh and refers to the time of his birth and after. Was he a real man? Was he truly God? How was the Word, which was in the beginning before all things, made flesh (John 1:14)?
The Biblical teaching of the person of Christ is clear and simple:
There are many heresies, but they can all be summarized by a denial of one of the above statements. Changing these statements changes who Christ is and fundamentally alters the foundation of Christianity.
Denying Christ is Man
Heresies that deny Christ’s humanity are not popular today, though you can still see them float around if you wait long enough. For example, Jehovah Witnesses’ teach that Jesus’ resurrection was not in a real body which would be a denial of his full humanity.
An ancient heresy called Docetism said that Jesus was God, but that he only appeared to be a man. Apollinarian heresy said that he had a human body, but a divine soul. Since a man is made of both body and soul, not having the soul of a man makes him an incomplete man.
Errors in this area most often arise from the tendency to protect his deity, or a misunderstanding of what it means to be a man (e.g. thinking being a man requires sin). All of these heresies teach a different Christ than what is found when all scripture is considered.
Jesus was fully and completely man in order to die for humanity, and to quicken any man to his image in salvation after death. He was in all ways like we are, human, yet without sin (Heb 4:15).
Denying Christ is God
A more popular and still prevalent heresy is denying the deity of Christ.
Unitarians, Jehovah Witnesses (modern day Arian heresy), Christadelphians, and other groups teach that Jesus was not God in the ultimate sense, but was a created being.
They vary in the degree to which Christ is magnified, but all deny Jesus as God. Many scholars of religion, Jews, and secularists fall into this category acknowledging the full humanity of Jesus, but denying he was God (modern day Ebionite heresy).
Adoptionism says that Jesus was a man and put on divinity at his baptism or his resurrection. The problem here is clear when we consider God’s attributes of eternality and necessity. If Jesus was not God for a single moment, he could not be God at any time, because God is forever.
The logic of scripture demands either that he was always God or never God.
If Jesus was not God, the Bible is wrong and Christianity is false. If Christ was not God then he could not atone for our sins, forgive a single sin, God would not be the Saviour, our faith would be in vain, and eternal life would be a wish not a promise.
Denying He is Both at the Same Time
One of the most subtle heresies and prevalent in otherwise sound Christian groups is the heresy regarding the union of the two natures of Christ (i.e. hypostatic union).
The true teaching is that Jesus is one eternal person who, after the incarnation, is described by two complete, unaltered, and unmixed natures: God and man. (Before the incarnation he was only God).
These kind of heresies are created when people allow one of the natures of Christ to change/affect/alter/limit/diminish the other nature.
Eutychianism and monophysitism are ancient heresies that taught that Jesus existed in only one nature: a mixture of man and God. A sort of superman – half-man half-God. If this were true, then, like Superman, he was not a true human, but a mutant human, and he was not truly God, but a new form of God.
Nestorianism on the other heretical hand says that there were two persons of Jesus: Jesus the God and Jesus the Man. This provides for two natures, but creates disunity in the person of Jesus who has always been one.
Kenoticism is a prevalent liberal heresy that is making inroads into more conservative Christian groups, and even in grace circles today. It teaches that in order to become a man, Jesus had to limit/diminish/empty/lay aside/subtract or in some way change his deity attributes.
If the two natures do not remain complete, full, inseparable, and without mixture, then either the Godhead must change, or Christ is not adequate to be the Saviour and mediator of men. Neither option is acceptable to Biblical Christianity, but that is why it is called heresy.
When the eternal divine Word was made flesh, he did not stop being God. He started also being human at the same time. From that point forward he was fully God with every attribute of deity, and he was fully man with every attribute of humanity. Jesus Christ is both man and God in one person.
Heresies are like poison and when ingested begin to disrupt the inner workings of the body. When taught and believed they affect how you think about God and salvation. If not now, then later when the crack in the foundation has had time to do its work.
Unlike other teachings which may be trivial or inconsequential to your salvation, getting the doctrine of Christ wrong throws into question whether you are a true Christian or not. If we teach another Jesus, then it is not the true Jesus Christ who is God and Saviour, Head of the Body, and in whom we are complete.
It is important to know the true teaching of Christ, and beware of the heresies throughout history that have often deceived men by fine words against the truth. Truth about Christ is necessary to be a true Christian.