From the LCMS website:
The first Council of Nicaea was convened in the early summer of 325 by the Roman Emperor Constantine at what is today Isnuk, Turkey. The emperor presided at the opening of the council. The council ruled against the Arians, who taught that Jesus was not the eternal Son of God but was created by the Father and was called Son of God because of his righteousness. The chief opponents of the Arians were Alexander, bishop of Alexandria, and his deacon, Athanasius. The council confessed the eternal divinity of Jesus and adopted the earliest version of the Nicene Creed, which in its entirety was adopted at the Council of Constantinople in 381.