The First Council of Constantinople was convened in 381 AD by Emperor Theodosius I in the city of Constantinople (now Istanbul, Turkey). It is considered the second ecumenical council. This council expanded and clarified the Nicene Creed, especially in reference to the Holy Spirit, declaring that the Holy Spirit is fully divine and equal with the Father and the Son. It thereby settled disputes about the divinity of the Holy Spirit and further denounced Arianism—the teaching that Christ was lesser than God the Father. The decisions of this council reinforced the doctrine of the Trinity and established Constantinople as the second in importance among the patriarchal sees, behind Rome (Britannica)[https://www.britannica.com/event/First-Council-of-Constantinople-381], (Wikipedia)[https://en.wikipedia.org/wiki/First_Council_of_Constantinople].
The Council of Chalcedon, held in 451 in the city of Chalcedon (near modern-day Istanbul), was the fourth ecumenical council of the Christian Church. Over 520 bishops or their representatives attended, making it the largest and best-documented of the early councils. The most significant outcome was its definition of the nature of Christ: the council affirmed that Jesus is to be acknowledged in two natures, “without confusion, without change, without division, without separation”—meaning Christ is fully God and fully human, united in one person. This became known as the “Chalcedonian Definition,” and it established orthodox doctrine on Christ’s nature for much of Christianity. The council also condemned the monophysite position (that Christ had only one divine nature) and ratified the creeds of both Nicaea and Constantinople, reinforcing the foundational teachings of earlier councils (Britannica)[https://www.britannica.com/event/Council-of-Chalcedon], (Wikipedia)[https://en.wikipedia.org/wiki/Council_of_Chalcedon].