Heresy is a denial of some truth by a person who is baptised and later professes disbelief in that truth.
Gnosticism is an example of heresy and is the earliest recorded example. This heresy developed as people who believed in Jesus began to deny the humanity of Jesus. They believed the body is unholy and therefore Jesus could not have a human body. Thus they denied the Incarnation of Jesus born of Mary.
Arianisn, which is named for Arius who was an Alexandrian priest, taught Jesus was not God because he was human. He and his followers taught Jesus was not the Son of God.
Nestorianism taught the Son of God lived in the body of a man, Jesus, who lived in Nazareth. Their teaching was Mary the Mother of Jesus was not the Mother of God, because Jesus and the Son of God were two separate persons.