As fully divine and fully human, Jesus must have had a human soul, right? When did that soul come to exist, and what happened to it after Jesus died on the cross? Did it cease to exist?
Yes, Jesus had a human soul during his life on earth, the same soul he continues to have in his glorified body in heaven. He was and remains fully human and fully divine. If he now lacked a human soul, he could not be fully divine and fully human, which is what we celebrate at Christmas.
Once the Roman persecutions ended in the fourth century AD, controversies within the Church centered on how to talk about Jesus and later the Holy Spirit. What we know as the Nicene Creed began to clarify the situation: Jesus is of the same nature as God the Father. The teaching of Arius, who denied that, was rejected.
In 381, another ecumenical council was held in Constantinople. The bishops there added a final section about the Holy Spirit. The Council of Chalcedon (451) formulated the basic teaching that Orthodox, Catholic, and mainline Protestant Christians still accept.