Can you please give me a list of all ecumenical councils and summarize what the core issues that needed to be discussed that time and what was decided at each of them
user
The first ecumenical council was held in Nicaea in 325. 
    The second ecumenical council was held in Constantinople in 381.
    The third ecumenical council was held in Ephesus in 431. 
    The fourth ecumenical council was held in Chalcedon in 451. 
    The fifth ecumenical council was held in Constantinople in 553. 
    The sixth ecumenical council was held in Constantinople in 680.  
    The seventh ecumenical council was held in Nicaea in 787. 

There have been 7 ecumenical councils in total. The first four were called into being by the Roman Emperor Constantine I and were focused on theological issues such as Arianism, which is a Christian belief that Jesus Christ is subordinate to God the Father and not equal in power or eternity. The next three (the Fifth, Sixth, and Seventh) were called into being by the Byzantine Empire’s Justinian I and his successors.
assistant
What was decided at the first ecumenical council, if that is known?
user
The first ecumenical council was the First Council of Nicaea, which was held in Nicaea (modern-day Iznik, Turkey) in 325 AD. It was convened by the Roman Emperor Constantine I to address the issue of Arianism, a controversial teaching that denied the divinity of Jesus Christ.

At the council, the bishops declared that Jesus Christ was of the same substance (homoousios) as God the Father, and condemned the Arian teaching that Jesus was a created being. This doctrine of the Trinity, which asserts that there is one God in three persons (the Father, the Son, and the Holy Spirit), became the orthodox Christian teaching and is still held by most Christians today.

The council also established a uniform date for celebrating Easter and issued other canons related to church discipline, such as the ordination of bishops and the treatment of heretics.

The decisions made at the First Council of Nicaea were highly influential in shaping Christian theology and doctrine for centuries to come.
assistant