What major theological issue did the Council of Ephesus deal with?

The Council of Ephesus, convened in 431 AD, primarily addressed the critical theological issue of the legitimacy of Christ's divine motherhood. This council was significant in affirming the title of Theotokos for Mary, which means "God-bearer" or "Mother of God." This title was fiercely defended against the views of Nestorius, who suggested that Mary should be called Christotokos, meaning "Christ-bearer," implying a separation between Christ's humanity and divinity.

Affirming Mary as Theotokos upheld the doctrine of the hypostatic union, which teaches that Christ is one person with two natures, divine and human, fully united. This was essential to the understanding of Jesus' identity and the incarnation. The resolution of this debate at Ephesus had profound implications for Christology and the understanding of both Jesus and Mary within Christian theology.

Other theological matters like the existence of the Trinity, the nature of the Holy Spirit, and the resurrection of the dead were certainly important in Christian doctrine, but they were not the central focus of the Council of Ephesus. This council specifically tackled the issue surrounding Mary and her relationship to Christ in light of theological developments at the time.