Apollinarius tried to say that Jesus could not have sinned because his human nature had no will of its own. In doing this, he stumbled onto a heresy called Monothelitism ("one-will" christology), which would become a huge controversy later. But a Christ without a human will would be a Christ who is not fully human. He would only be wearing a human body like a costume, but he would not be truly human.
Links
To listen to Mike Aquilina's episode 30 on Gregory of Nazianzus: https://www.catholicculture.org/commentary/gregory-nazianzen-greatness-in-passive-voice/
To listen to Mike Aquilina's episode 2.3 on The Council of Constantinople: https://www.catholicculture.org/commentary/23-first-constantinople-capital-council/
To listen to Mike Aquilina's episode 54 on Maximus the Confessor: https://www.catholicculture.org/commentary/54maximus-confessor-where-east-and-west-meet/
To read Gregory of Nazianzus' First Letter to Cledonius Against Apollinarius: https://www.catholicculture.org/culture/library/fathers/view.cfm?recnum=2459&repos=8&subrepos=0&searchid=2402862
To read Gregory of Nazianzus' Second Letter to Cledonius Against Apollinarius: https://www.catholicculture.org/culture/library/fathers/view.cfm?recnum=2463&repos=8&subrepos=0&searchid=2402862
For more on the christological controversies in context, see the book:
Reading the Church Fathers: A History of the Early Church and the Development of Doctrine: https://sophiainstitute.com/product/reading-the-church-fathers/
For more on the concept of the will of God as it relates to God's sovereignty, human free will, and the submission of the human will to the will of God, see the book:
Praying Like the Early Church: Seven Insights from the Church Fathers to Help You Connect with God: https://sophiainstitute.com/product/praying-like-the-early-church/
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To connect with Dr. James Papandrea, On YouTube - The Original Church: https://www.youtube.com/@TheOriginalChurch
Join the conversation in the Original Church Community on Locals: https://theoriginalchurch.locals.com/
Dr. Papandrea's Homepage: http://www.jimpapandrea.com
Theme Music: Gaudeamus (Introit for the Feast of All Saints), sung by Jeff Ostrowski. Courtesy of Corpus Christi Watershed: https://www.ccwatershed.org/