may 1, 325 - First Council of Nicaea

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Its main accomplishments were settlement of the Christological issue of the divine nature of God the Son and his relationship to God the Father,[2] the construction of the first part of the Nicene Creed, establishing uniform observance of the date of Easter,[6] and promulgation of early canon law. Nicene Creed.

Constantine was in attendance.

https://en.wikipedia.org/wiki/First_Council_of_Nicaea
https://en.wikipedia.org/wiki/Nicene_Creed

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