Apollinarianism is a very long name for an early Christian heresy. It was declared a heresy first at 381 by the Council of Constantinople, and it was also dealt with at the Council of Chalcedon in 451.
Apollinarianism is named after the bishop Apollinaris. He was probably born in 310, though some think he was born as early as 300. He died in 390, so his life spanned almost the entire fourth century.
In 360, he was appointed bishop of Laodicea. This was a very important city with an important church, a church that had its roots all the way back in the pages of the New Testament. Apollinaris was bishop there until 375.
He was also friends with Athanasius. A crucial figure in the development of Christology, Athanasius fought valiantly for the doctrine that Christ is fully God and fully human. Athanasius’ foe was Arius, who denied the deity of Christ, and Apollinaris shared his concerns regarding Arianism.
Apollinaris, however, overreacted to the teachings of Arius and went too far in the other direction. Apollinaris taught that in Jesus, the divine Logos of John 1 replaces the human soul. So, Jesus the God-man is not fully human. He’s only a material man, and we know that humans are more than just material; they also have an immaterial part. Apollinaris rejected the full humanity of Christ to promote His full divinity.
This teaching was roundly rejected by the church. In 374, Apollinaris was dismissed from his post as bishop of Laodicea. We’re not sure what Apollinaris was doing from 374 to 390, but we do know that he heard the news of the Council of Constantinople in 381. This council met for two reasons. First, it needed to reaffirm the teachings of the Nicene Creed and the Nicene Council of 325. Since the Nicene Council, Arianism had once again made its way into the church and had influenced bishops and people in the church. Second, it needed to address Apollinarianism. One hundred and fifty bishops gathered in Constantinople and reaffirmed the Council of Nicaea and declared Apollinarianism a heresy. Jesus Christ is fully God and Jesus Christ is fully human. As the author of Hebrews tells us, “He had to be made like his brothers” (Heb. 2:17).