What key issue did the Council of Constantinople I address?

The Council of Constantinople I, which took place in 381, primarily dealt with the divinity of the Holy Spirit. This council was significant in affirming the doctrine of the Trinity, which posits that the Holy Spirit is fully divine and of the same essence as the Father and the Son. One of the major outcomes was the formulation of the Nicene Creed, which was expanded to include a clearer statement on the Holy Spirit, stating that He is worshipped and glorified together with the Father and the Son.

The context of the council was to address various heresies that challenged the understanding of the Trinity, and the affirmation of the Holy Spirit's divinity was crucial for the ongoing development of Christian theology. This decision helped to unify the Church's teachings and provided a clearer doctrinal framework that would guide Christian belief and practice in the years to come.